Add FailoverSwitchManager to handle provider switching after successful
failover. This ensures the UI reflects the actual provider in use:
- Create failover_switch.rs with deduplication and async switching logic
- Pass AppHandle through ProxyService -> ProxyServer -> RequestForwarder
- Update is_current in database when failover succeeds
- Emit provider-switched event for frontend refresh
- Update tray menu and live backup synchronously
The switching runs asynchronously via tokio::spawn to avoid blocking
API responses while still providing immediate UI feedback.
This commit addresses several critical issues in the failover system:
**Circuit breaker state persistence (previous fix)**
- Promote ProviderRouter to ProxyState for cross-request state sharing
- Remove redundant router.rs module
- Fix 429 errors to be retryable (rate limiting should try other providers)
**Hot-update circuit breaker config**
- Add update_circuit_breaker_configs() to ProxyServer and ProxyService
- Connect update_circuit_breaker_config command to running circuit breakers
- Add reset_provider_circuit_breaker() for manual breaker reset
**Fix HalfOpen deadlock bug**
- Change half_open_requests from cumulative count to in-flight count
- Release quota in record_success()/record_failure() when in HalfOpen state
- Prevents permanent deadlock when success_threshold > 1
**Fix duplicate select_providers() call**
- Store providers list in RequestContext, pass to forward_with_retry()
- Avoid consuming HalfOpen quota twice per request
- Single call to select_providers() per request lifecycle
**Add per-provider retry with exponential backoff**
- Implement forward_with_provider_retry() with configurable max_retries
- Backoff delays: 100ms, 200ms, 400ms, etc.
Extract common request handling logic into dedicated modules:
- handler_config.rs: Usage parser configurations for each API type
- handler_context.rs: Request lifecycle context management
- response_processor.rs: Unified streaming/non-streaming response handling
Reduces handlers.rs from ~1130 lines to ~418 lines (-63%), eliminating
repeated initialization and response processing patterns across the
four API handlers (Claude, Codex Chat, Codex Responses, Gemini).
Jason