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/core/src/session/mod.rs

@@ -3102,7 +3102,7 @@ impl Session {
             prepared_recommendations,
         )
         .or_cancel(cancellation_token)
-        .await?;
+        .await??;
         Ok(Arc::new(StepContext {
             turn: turn_context,
             environments,