fix(amp): enable OAuth fallback for Gemini v1beta1 routes

AMP CLI sends Gemini requests to non-standard paths that were being
directly proxied to ampcode.com without checking for local OAuth.

This fix adds:
- GeminiBridge handler to transform AMP CLI paths to standard format
- Enhanced model extraction from AMP's /publishers/google/models/* paths
- FallbackHandler wrapper to check for local OAuth before proxying

Flow:
- If user has local Google OAuth → use it (free tier)
- If no local OAuth → fallback to ampcode.com (charges credits)

Fixes issue where gemini-3-pro-preview requests always charged AMP
credits even when user had valid Google Cloud OAuth configured.

internal/api/modules/amp/gemini_bridge.go

@@ -0,0 +1,45 @@
+package amp
+
+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
+// to our standard Gemini handler by rewriting the request context.
+//
+// AMP CLI format: /publishers/google/models/gemini-3-pro-preview:streamGenerateContent
+// Standard format: /models/gemini-3-pro-preview:streamGenerateContent
+//
+// 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 {
+	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/"
+
+			// Set this as the :action parameter that the Gemini handler expects
+			c.Params = append(c.Params, gin.Param{
+				Key:   "action",
+				Value: actionPart,
+			})
+
+			// Call the standard Gemini handler
+			geminiHandler.GeminiHandler(c)
+			return
+		}
+
+		// If we can't parse the path, return 400
+		c.JSON(400, gin.H{
+			"error": "Invalid Gemini API path format",
+		})
+	}
+}