Fixes thinking signature validation errors

Addresses an issue where thinking signature validation fails due to model mapping and empty internal registry.

- Implements a fallback mechanism in the router to use the global model registry when the internal registry is empty. This ensures that models registered via API keys are correctly resolved even without local provider configurations.
- Modifies `GetModelGroup` to use registry-based grouping in addition to name pattern matching, covering cases where models are registered with API keys but lack provider names in their names.
- Updates signature validation to compare model groups instead of exact model names.

These changes resolve thinking signature validation errors and improve the accuracy of model resolution.

internal/cache/signature_cache.go

@@ -6,6 +6,8 @@ import (
 	"strings"
 	"sync"
 	"time"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/registry"
 )
 
 // SignatureEntry holds a cached thinking signature with timestamp
@@ -184,6 +186,7 @@ func HasValidSignature(modelName, signature string) bool {
 }
 
 func GetModelGroup(modelName string) string {
+	// Fast path: check model name patterns first
 	if strings.Contains(modelName, "gpt") {
 		return "gpt"
 	} else if strings.Contains(modelName, "claude") {
@@ -191,5 +194,21 @@ func GetModelGroup(modelName string) string {
 	} else if strings.Contains(modelName, "gemini") {
 		return "gemini"
 	}
+
+	// Slow path: check registry for provider-based grouping
+	// This handles models registered via claude-api-key, gemini-api-key, etc.
+	// that don't have provider name in their model name (e.g., kimi-k2.5 via claude-api-key)
+	if providers := registry.GetGlobalRegistry().GetModelProviders(modelName); len(providers) > 0 {
+		provider := strings.ToLower(providers[0])
+		switch provider {
+		case "claude":
+			return "claude"
+		case "gemini", "gemini-cli", "aistudio", "vertex", "antigravity":
+			return "gemini"
+		case "codex":
+			return "gpt"
+		}
+	}
+
 	return modelName
 }