feat(claude): add thinking model variants and beta headers support

- Add Claude thinking model definitions (sonnet-4-5-thinking, opus-4-5-thinking variants)
- Add Thinking support for antigravity models with -thinking suffix
- Add injectThinkingConfig() for automatic thinking budget based on model suffix
- Add resolveUpstreamModel() mappings for thinking variants to actual Claude models
- Add extractAndRemoveBetas() to convert betas array to anthropic-beta header
- Update applyClaudeHeaders() to merge custom betas from request body

Closes #324

internal/runtime/executor/antigravity_executor.go

@@ -365,7 +365,7 @@ func FetchAntigravityModels(ctx context.Context, auth *cliproxyauth.Auth, cfg *c
 		for id := range result.Map() {
 			id = modelName2Alias(id)
 			if id != "" {
-				models = append(models, &registry.ModelInfo{
+				modelInfo := &registry.ModelInfo{
 					ID:          id,
 					Name:        id,
 					Description: id,
@@ -375,7 +375,17 @@ func FetchAntigravityModels(ctx context.Context, auth *cliproxyauth.Auth, cfg *c
 					Created:     now,
 					OwnedBy:     antigravityAuthType,
 					Type:        antigravityAuthType,
-				})
+				}
+				// Add Thinking support for thinking models
+				if strings.HasSuffix(id, "-thinking") || strings.Contains(id, "-thinking-") {
+					modelInfo.Thinking = &registry.ThinkingSupport{
+						Min:            1024,
+						Max:            100000,
+						ZeroAllowed:    false,
+						DynamicAllowed: true,
+					}
+				}
+				models = append(models, modelInfo)
 			}
 		}
 		return models