Add Codex Chat-to-Responses bridge

- Add Codex provider API format selection and model mapping for Chat-only upstreams.
- Convert Codex Responses requests to Chat Completions and rebuild Chat responses as Responses output.
- Preserve reasoning_content across non-streaming, streaming, tool calls, and previous_response_id follow-ups.
- Add a bounded Codex Chat history cache for restoring tool calls before tool outputs.
- Cover Chat bridge, compact routing, streaming, and history recovery with focused tests.

src-tauri/src/proxy/handler_context.rs

@@ -223,6 +223,7 @@ impl RequestContext {
             state.status.clone(),
             state.current_providers.clone(),
             state.gemini_shadow.clone(),
+            state.codex_chat_history.clone(),
             state.failover_manager.clone(),
             state.app_handle.clone(),
             self.current_provider_id.clone(),