feat(routing): implement unified model routing with OAuth and API key providers

- Added a new routing package to manage provider registration and model resolution.
- Introduced Router, Executor, and Provider interfaces to handle different provider types.
- Implemented OAuthProvider and APIKeyProvider to support OAuth and API key authentication.
- Enhanced DefaultModelMapper to include OAuth model alias handling and fallback mechanisms.
- Updated context management in API handlers to preserve fallback models.
- Added tests for routing logic and provider selection.
- Enhanced Claude request conversion to handle reasoning content based on thinking mode.

internal/routing/ctxkeys/keys.go

@@ -0,0 +1,11 @@
+package ctxkeys
+
+type key string
+
+const (
+	MappedModel     key = "mapped_model"
+	FallbackModels  key = "fallback_models"
+	RouteCandidates key = "route_candidates"
+	RoutingDecision key = "routing_decision"
+	MappingApplied  key = "mapping_applied"
+)