refactor: improve gemini bridge testability and code quality

- Change createGeminiBridgeHandler to accept gin.HandlerFunc instead of *gemini.GeminiAPIHandler
  This allows tests to inject mock handlers instead of duplicating bridge logic
- Replace magic number 8 with len(modelsPrefix) for better maintainability
- Remove redundant test case that doesn't test edge case in production
- Update routes.go to pass geminiHandlers.GeminiHandler directly

Addresses PR review feedback on test architecture and code clarity.

Amp-Thread-ID: https://ampcode.com/threads/T-1ae2c691-e434-4b99-a49a-10cabd3544db

internal/api/modules/amp/gemini_bridge.go

@@ -4,7 +4,6 @@ import (
 	"strings"
 
 	"github.com/gin-gonic/gin"
-	"github.com/router-for-me/CLIProxyAPI/v6/sdk/api/handlers/gemini"
 )
 
 // createGeminiBridgeHandler creates a handler that bridges AMP CLI's non-standard Gemini paths
@@ -15,16 +14,19 @@ import (
 //
 // This extracts the model+method from the AMP path and sets it as the :action parameter
 // so the standard Gemini handler can process it.
-func createGeminiBridgeHandler(geminiHandler *gemini.GeminiAPIHandler) gin.HandlerFunc {
+//
+// The handler parameter should be a Gemini-compatible handler that expects the :action param.
+func createGeminiBridgeHandler(handler gin.HandlerFunc) gin.HandlerFunc {
 	return func(c *gin.Context) {
 		// Get the full path from the catch-all parameter
 		path := c.Param("path")
 
 		// Extract model:method from AMP CLI path format
 		// Example: /publishers/google/models/gemini-3-pro-preview:streamGenerateContent
-		if idx := strings.Index(path, "/models/"); idx >= 0 {
-			// Extract everything after "/models/"
-			actionPart := path[idx+8:] // Skip "/models/"
+		const modelsPrefix = "/models/"
+		if idx := strings.Index(path, modelsPrefix); idx >= 0 {
+			// Extract everything after modelsPrefix
+			actionPart := path[idx+len(modelsPrefix):]
 
 			// Check if model was mapped by FallbackHandler
 			if mappedModel, exists := c.Get(MappedModelContextKey); exists {
@@ -44,8 +46,8 @@ func createGeminiBridgeHandler(geminiHandler *gemini.GeminiAPIHandler) gin.Handl
 				Value: actionPart,
 			})
 
-			// Call the standard Gemini handler
-			geminiHandler.GeminiHandler(c)
+			// Call the handler
+			handler(c)
 			return
 		}