[codex] Preserve cache_control when merging system prompts (#1946)

* Preserve cache hints when collapsing system prompts

Strict OpenAI-compatible chat backends still need fragmented Claude\nsystem prompts collapsed into one leading system message, but that\nnormalization should not silently drop stable cache hints. Preserve\nmessage-level cache_control when the merged system fragments agree,\nand fall back to omitting it when the fragments conflict.\n\nConstraint: Must keep single-system normalization for Nvidia/Qwen-style chat backends\nRejected: Always copy the first cache_control | could misrepresent conflicting cache boundaries\nConfidence: high\nScope-risk: narrow\nReversibility: clean\nDirective: If system prompt merging changes again, preserve cache_control whenever the merged metadata is unambiguous\nTested: cargo test proxy::providers::transform --manifest-path src-tauri/Cargo.toml\nNot-tested: End-to-end prompt caching behavior against cache-aware OpenAI-compatible upstreams\nRelated: #1881

* Tighten cache hint inheritance for merged system prompts

The follow-up cache hint fix still treated mixed present/absent\ncache_control across fragmented system prompts as inheritable, which\nexpanded the cache scope after prompt collapse. Treat that mix as\nambiguous and only preserve cache_control when every merged fragment\nexplicitly agrees on the same value.\n\nConstraint: Must preserve strict-backend system prompt normalization from #1942\nRejected: Inherit first present cache_control | widens cache scope when later fragments were intentionally uncached\nConfidence: high\nScope-risk: narrow\nReversibility: clean\nDirective: Any future merged-system cache hint logic should treat missing cache_control as semantically significant\nTested: cargo test proxy::providers::transform --manifest-path src-tauri/Cargo.toml\nNot-tested: End-to-end upstream caching behavior against cache-aware relays\nRelated: #1881\nRelated: #1946

* Keep cache-control merge regressions easy to review

Reflow the two long cache-control regression assertions in transform.rs so the neighboring merge cases stay rustfmt-aligned and easier to scan.

This keeps the preserved code change separate from the untracked Markdown design notes the user did not want committed.

Constraint: Exclude Markdown design files from the commit while preserving the local code change
Rejected: Include docs in the same commit | user explicitly asked to leave Markdown files out
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Treat this as a readability-only test change; do not infer runtime behavior changes from it
Tested: cargo test --manifest-path src-tauri/Cargo.toml test_anthropic_to_openai_drops_ --lib
Tested: cargo check --manifest-path src-tauri/Cargo.toml --tests
Tested: pnpm format:check
Tested: pnpm typecheck
Not-tested: Full application integration and manual flows

src-tauri/src/proxy/providers/transform.rs

@@ -199,6 +199,10 @@ fn normalize_openai_system_messages(messages: &mut Vec<Value>) {
     }
 
     let mut parts = Vec::new();
+    let mut inherited_cache_control: Option<Value> = None;
+    let mut cache_control_conflict = false;
+    let mut saw_cache_control = false;
+    let mut saw_missing_cache_control = false;
     messages.retain(|message| {
         if message.get("role").and_then(|value| value.as_str()) != Some("system") {
             return true;
@@ -219,11 +223,28 @@ fn normalize_openai_system_messages(messages: &mut Vec<Value>) {
             _ => {}
         }
 
+        if let Some(cache_control) = message.get("cache_control") {
+            saw_cache_control = true;
+            match &inherited_cache_control {
+                None => inherited_cache_control = Some(cache_control.clone()),
+                Some(existing) if existing == cache_control => {}
+                Some(_) => cache_control_conflict = true,
+            }
+        } else {
+            saw_missing_cache_control = true;
+        }
+
         false
     });
 
     if !parts.is_empty() {
-        messages.insert(0, json!({"role": "system", "content": parts.join("\n")}));
+        let mut merged = json!({"role": "system", "content": parts.join("\n")});
+        if !(cache_control_conflict || (saw_cache_control && saw_missing_cache_control)) {
+            if let Some(cache_control) = inherited_cache_control {
+                merged["cache_control"] = cache_control;
+            }
+        }
+        messages.insert(0, merged);
     }
 }
 
@@ -606,18 +627,15 @@ mod tests {
     }
 
     #[test]
-    fn test_anthropic_to_openai_normalizes_fragmented_system_messages() {
+    fn test_anthropic_to_openai_preserves_matching_system_cache_control_when_merging() {
         let input = json!({
             "model": "claude-3-sonnet",
             "max_tokens": 1024,
             "system": [
-                {"type": "text", "text": "You are Claude Code."},
-                {"type": "text", "text": "Be concise."}
+                {"type": "text", "text": "You are Claude Code.", "cache_control": {"type": "ephemeral"}},
+                {"type": "text", "text": "Be concise.", "cache_control": {"type": "ephemeral"}}
             ],
-            "messages": [
-                {"role": "system", "content": "Follow repo conventions."},
-                {"role": "user", "content": "Hello"}
-            ]
+            "messages": [{"role": "user", "content": "Hello"}]
         });
 
         let result = anthropic_to_openai(input).unwrap();
@@ -625,11 +643,54 @@ mod tests {
         assert_eq!(result["messages"][0]["role"], "system");
         assert_eq!(
             result["messages"][0]["content"],
-            "You are Claude Code.\nBe concise.\nFollow repo conventions."
+            "You are Claude Code.\nBe concise."
         );
+        assert_eq!(result["messages"][0]["cache_control"]["type"], "ephemeral");
         assert_eq!(result["messages"][1]["role"], "user");
     }
 
+    #[test]
+    fn test_anthropic_to_openai_drops_mixed_present_absent_system_cache_control_when_merging() {
+        let input = json!({
+            "model": "claude-3-sonnet",
+            "max_tokens": 1024,
+            "system": [
+                {"type": "text", "text": "You are Claude Code.", "cache_control": {"type": "ephemeral"}},
+                {"type": "text", "text": "Be concise."}
+            ],
+            "messages": [{"role": "user", "content": "Hello"}]
+        });
+
+        let result = anthropic_to_openai(input, None).unwrap();
+        assert_eq!(result["messages"][0]["role"], "system");
+        assert_eq!(
+            result["messages"][0]["content"],
+            "You are Claude Code.\nBe concise."
+        );
+        assert!(result["messages"][0].get("cache_control").is_none());
+    }
+
+    #[test]
+    fn test_anthropic_to_openai_drops_conflicting_system_cache_control_when_merging() {
+        let input = json!({
+            "model": "claude-3-sonnet",
+            "max_tokens": 1024,
+            "system": [
+                {"type": "text", "text": "You are Claude Code.", "cache_control": {"type": "ephemeral"}},
+                {"type": "text", "text": "Be concise.", "cache_control": {"type": "ephemeral", "ttl": "5m"}}
+            ],
+            "messages": [{"role": "user", "content": "Hello"}]
+        });
+
+        let result = anthropic_to_openai(input, None).unwrap();
+        assert_eq!(result["messages"][0]["role"], "system");
+        assert_eq!(
+            result["messages"][0]["content"],
+            "You are Claude Code.\nBe concise."
+        );
+        assert!(result["messages"][0].get("cache_control").is_none());
+    }
+
     #[test]
     fn test_anthropic_to_openai_tool_use() {
         let input = json!({