Fix issues from PR #613 review feedback

- Fix start() holding mutex across FFI call: release lock before
  native core call, add 'starting' transitional state to prevent
  concurrent start() races (mirrors Rust PR feedback about
  mutex-across-channel-send deadlock)
- Remove codex.md review artifact from repo (reviewer feedback)
- Expose active_settings() getter for frozen settings snapshot
- Break stop() into drain_push_queue_and_thread(), send_native_stop(),
  enqueue_final_result() helpers for readability
- Improve error messages with more context
- Add 2 new tests: StartError_RevertsToNotStarted_CanRetry,
  ActiveSettings_ExposedAfterStart
- Update SettingsFrozenAfterStart test to verify via active_settings()
- All 62 tests passing (59 unit + 3 E2E)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: ruiren_microsoft

sdk/cpp/codex.md

@@ -99,12 +99,21 @@ class LiveAudioTranscriptionSession {
     /// Check if the session has been stopped.
     bool is_stopped() const;
 
+    /// Get the frozen settings snapshot that was active when start() was called.
+    /// Only valid after start() has been called.
+    const LiveAudioTranscriptionOptions& active_settings() const { return active_settings_; }
+
 private:
-    enum class State { not_started, started, stopping, stopped, failed };
+    enum class State { not_started, starting, started, stopping, stopped, failed };
 
     void push_loop();
     void transition_to(State new_state);
 
+    // Helpers extracted from stop() for readability
+    void drain_push_queue_and_thread();
+    detail::CoreInteropResponse send_native_stop(const std::string& handle);
+    void enqueue_final_result(const detail::CoreInteropResponse& response);
+
     std::string model_id_;
     std::shared_ptr<detail::CoreInterop> core_interop_;
 
@@ -114,7 +123,7 @@ class LiveAudioTranscriptionSession {
     std::string session_handle_;
     std::string error_message_;
 
-    // Frozen settings snapshot
+    // Frozen settings snapshot (exposed via active_settings())
     LiveAudioTranscriptionOptions active_settings_;
 
     // User-configurable settings (frozen at start)