Propagate thread id in MCP tool metadata (#18093)

## Summary
- attach the authoritative Codex thread id to MCP tool request
`_meta.threadId` for model-initiated tool calls
- attach the same thread id for manual `mcpServer/tool/call` requests
before invoking the MCP server
- cover both metadata helper behavior and the manual app-server MCP path
in tests


needed because the Rust app-server is the last place that still has
authoritative knowledge of “this model-generated MCP tool call belongs
to conversation/thread X” before the request leaves Codex and reaches
Hoopa. It adds threadId to MCP request metadata in the model-generated
tool-call path, using sess.conversation_id, and also does the same for
the manual mcpServer/tool/call path.

## Test plan
- `cargo test -p codex-core
mcp_tool_call_thread_id_meta_is_added_to_request_meta --lib`
- `cargo test -p codex-app-server
mcp_server_tool_call_returns_tool_result`

Paired Hoopa consumer PR: https://github.com/openai/openai/pull/833263

codex-rs/core/src/mcp_tool_call.rs

@@ -477,6 +477,8 @@ async fn execute_mcp_tool_call(
         metadata.and_then(|metadata| metadata.openai_file_input_params.as_deref()),
     )
     .await?;
+    let request_meta =
+        with_mcp_tool_call_thread_id_meta(request_meta, &sess.conversation_id.to_string());
     let request_meta =
         augment_mcp_tool_request_meta_with_sandbox_state(sess, turn_context, server, request_meta)
             .await
@@ -660,6 +662,7 @@ pub(crate) struct McpToolApprovalMetadata {
 const MCP_TOOL_CODEX_APPS_META_KEY: &str = "_codex_apps";
 const MCP_TOOL_OPENAI_OUTPUT_TEMPLATE_META_KEY: &str = "openai/outputTemplate";
 const MCP_TOOL_UI_RESOURCE_URI_META_KEY: &str = "ui/resourceUri";
+const MCP_TOOL_THREAD_ID_META_KEY: &str = "threadId";
 
 fn custom_mcp_tool_approval_mode(
     turn_context: &TurnContext,
@@ -714,6 +717,30 @@ fn build_mcp_tool_call_request_meta(
     (!request_meta.is_empty()).then_some(serde_json::Value::Object(request_meta))
 }
 
+fn with_mcp_tool_call_thread_id_meta(
+    meta: Option<serde_json::Value>,
+    thread_id: &str,
+) -> Option<serde_json::Value> {
+    match meta {
+        Some(serde_json::Value::Object(mut map)) => {
+            map.insert(
+                MCP_TOOL_THREAD_ID_META_KEY.to_string(),
+                serde_json::Value::String(thread_id.to_string()),
+            );
+            Some(serde_json::Value::Object(map))
+        }
+        None => {
+            let mut map = serde_json::Map::new();
+            map.insert(
+                MCP_TOOL_THREAD_ID_META_KEY.to_string(),
+                serde_json::Value::String(thread_id.to_string()),
+            );
+            Some(serde_json::Value::Object(map))
+        }
+        other => other,
+    }
+}
+
 #[derive(Clone, Copy)]
 struct McpToolApprovalPromptOptions {
     allow_session_remember: bool,

codex-rs/core/src/mcp_tool_call_tests.rs

@@ -651,6 +651,35 @@ async fn codex_apps_tool_call_request_meta_includes_turn_metadata_and_codex_apps
     );
 }
 
+#[test]
+fn mcp_tool_call_thread_id_meta_is_added_to_request_meta() {
+    assert_eq!(
+        with_mcp_tool_call_thread_id_meta(
+            Some(serde_json::json!({
+                "source": "test-client",
+                "threadId": "stale-thread",
+            })),
+            "thread-live",
+        ),
+        Some(serde_json::json!({
+            "source": "test-client",
+            "threadId": "thread-live",
+        }))
+    );
+
+    assert_eq!(
+        with_mcp_tool_call_thread_id_meta(/*meta*/ None, "thread-live"),
+        Some(serde_json::json!({
+            "threadId": "thread-live",
+        }))
+    );
+
+    assert_eq!(
+        with_mcp_tool_call_thread_id_meta(Some(serde_json::json!("invalid-meta")), "thread-live"),
+        Some(serde_json::json!("invalid-meta"))
+    );
+}
+
 #[test]
 fn accepted_elicitation_content_converts_to_request_user_input_response() {
     let response = request_user_input_response_from_elicitation_content(Some(serde_json::json!(