fix(proxy): resolve HalfOpen counter underflow and config field inconsistencies

- Fix HalfOpen counter underflow: increment half_open_requests when
  transitioning from Open to HalfOpen to prevent underflow in
  record_success/record_failure

- Fix Gemini config field names: unify to GEMINI_API_KEY and
  GOOGLE_GEMINI_BASE_URL (removed GOOGLE_API_KEY and GEMINI_API_BASE)

- Fix Codex proxy takeover: write base_url to config.toml instead of
  OPENAI_BASE_URL in auth.json (Codex CLI reads from config.toml)

src-tauri/src/proxy/circuit_breaker.rs

@@ -115,6 +115,8 @@ impl CircuitBreaker {
                             "Circuit breaker transitioning from Open to HalfOpen (timeout reached)"
                         );
                         self.transition_to_half_open().await;
+                        // 增加计数，确保 record_success/record_failure 减计数时不会下溢
+                        self.half_open_requests.fetch_add(1, Ordering::SeqCst);
                         return true;
                     }
                 }