[codex] Filter legacy warning messages during compaction (#22243)

## Why

Older sessions can contain model-warning records persisted as `user`
messages, including the unified exec process-limit warning, the
`apply_patch`-via-`exec_command` warning, and the model-mismatch
high-risk cyber fallback warning. Those warnings are no longer produced
as conversation history items, but when old sessions compact they should
still be recognized as injected context rather than preserved as real
user turns.

## What changed

- Removed `record_model_warning` and the production paths that emitted
these warning messages into conversation history.
- Added `LegacyUnifiedExecProcessLimitWarning`,
`LegacyApplyPatchExecCommandWarning`, and `LegacyModelMismatchWarning`
contextual fragments that are used only for matching old persisted
messages.
- Registered the legacy fragments with contextual user message detection
so compaction filters them through the existing fragment path.
- Added focused compaction coverage for old warning messages being
dropped during compacted-history processing.

## Testing

- `cargo test -p codex-core warning`
- `just fix -p codex-core`

codex-rs/core/src/compact_tests.rs

@@ -23,6 +23,17 @@ async fn process_compacted_history_with_test_session(
     (refreshed, initial_context)
 }
 
+fn user_message(text: &str) -> ResponseItem {
+    ResponseItem::Message {
+        id: None,
+        role: "user".to_string(),
+        content: vec![ContentItem::InputText {
+            text: text.to_string(),
+        }],
+        phase: None,
+    }
+}
+
 #[test]
 fn content_items_to_text_joins_non_empty_segments() {
     let items = vec![
@@ -120,6 +131,26 @@ do things
     assert_eq!(vec!["real user message".to_string()], collected);
 }
 
+#[test]
+fn collect_user_messages_filters_legacy_warnings() {
+    let items = vec![
+        user_message(
+            "Warning: The maximum number of unified exec processes you can keep open is 60 and you currently have 61 processes open. Reuse older processes or close them to prevent automatic pruning of old processes",
+        ),
+        user_message(
+            "Warning: apply_patch was requested via exec_command. Use the apply_patch tool instead of exec_command.",
+        ),
+        user_message(
+            "Warning: Your account was flagged for potentially high-risk cyber activity and this request was routed to gpt-5.2 as a fallback. To regain access to gpt-5.3-codex, apply for trusted access: https://chatgpt.com/cyber or learn more: https://developers.openai.com/codex/concepts/cyber-safety",
+        ),
+        user_message("real user message"),
+    ];
+
+    let collected = collect_user_messages(&items);
+
+    assert_eq!(vec!["real user message".to_string()], collected);
+}
+
 #[test]
 fn build_token_limited_compacted_history_truncates_overlong_user_messages() {
     // Use a small truncation limit so the test remains fast while still validating
@@ -351,6 +382,31 @@ keep me updated
     assert_eq!(refreshed, expected);
 }
 
+#[tokio::test]
+async fn process_compacted_history_drops_legacy_warnings() {
+    let latest_user = user_message("latest user");
+    let compacted_history = vec![
+        user_message(
+            "Warning: The maximum number of unified exec processes you can keep open is 60 and you currently have 61 processes open. Reuse older processes or close them to prevent automatic pruning of old processes",
+        ),
+        user_message(
+            "Warning: apply_patch was requested via exec_command. Use the apply_patch tool instead of exec_command.",
+        ),
+        user_message(
+            "Warning: Your account was flagged for potentially high-risk cyber activity and this request was routed to gpt-5.2 as a fallback. To regain access to gpt-5.3-codex, apply for trusted access: https://chatgpt.com/cyber or learn more: https://developers.openai.com/codex/concepts/cyber-safety",
+        ),
+        latest_user.clone(),
+    ];
+    let (refreshed, initial_context) = process_compacted_history_with_test_session(
+        compacted_history,
+        /*previous_turn_settings*/ None,
+    )
+    .await;
+    let mut expected = initial_context;
+    expected.push(latest_user);
+    assert_eq!(refreshed, expected);
+}
+
 #[tokio::test]
 async fn process_compacted_history_inserts_context_before_last_real_user_message_only() {
     let compacted_history = vec![