Enforce strict tool name collision errors (#37020)

## What changed

- Track the first duplicate effective tool name while assembling the tool registry, including collisions introduced by external tools, code mode, and tool search.
- When `[features.tool_registry].error_on_tool_collisions` is enabled, fail the turn with a `duplicate tool: <namespace>.<name>` error before sending a model request.
- Continue allowing identical tool names in different namespaces, and preserve the existing collision behavior when strict checking is disabled.

## Testing

- Cover registry collision tracking, strict tool planning across tool sources, namespace isolation, failure before sampling, and pre-sampling compaction.

GitOrigin-RevId: 4cf83538fb513cec0b9c8b567780caaaadd3243d

codex-rs/features/src/lib.rs

@@ -654,9 +654,9 @@ pub fn canonical_feature_for_key(key: &str) -> Option<Feature> {
         .map(|spec| spec.id)
 }
 
-/// Returns `true` if the provided string matches a known feature toggle key.
+/// Returns `true` if the provided string matches a known `[features]` key.
 pub fn is_known_feature_key(key: &str) -> bool {
-    feature_for_key(key).is_some()
+    key == "tool_registry" || feature_for_key(key).is_some()
 }
 
 /// Deserializable features table for TOML.

codex-rs/features/src/tests.rs

@@ -40,7 +40,8 @@ fn tool_registry_config_is_not_a_feature_toggle() {
         })
     );
     assert!(features.entries().is_empty());
-    assert!(!crate::is_known_feature_key("tool_registry"));
+    assert!(crate::is_known_feature_key("tool_registry"));
+    assert_eq!(feature_for_key("tool_registry"), None);
 }
 
 #[test]