fix(gemini): strip native self prefixes before generateContent (#36141)

Strip `google/` and `gemini/` self-prefixes before native Gemini generateContent calls, and keep provider-normalization expectations aligned.

agent/gemini_native_adapter.py

@@ -41,6 +41,16 @@ DEFAULT_GEMINI_BASE_URL = "https://generativelanguage.googleapis.com/v1beta"
 GEMINI_DEFAULT_MAX_OUTPUT_TOKENS = 65535
 
 
+def bare_gemini_model_id(model: str) -> str:
+    """Strip Gemini's own provider prefix from an aggregator-style model id."""
+    name = (model or "").strip()
+    lowered = name.lower()
+    for prefix in ("google/", "gemini/"):
+        if lowered.startswith(prefix):
+            return name[len(prefix):].strip() or name
+    return name
+
+
 def is_native_gemini_base_url(base_url: str) -> bool:
     """Return True when the endpoint speaks Gemini's native REST API."""
     normalized = str(base_url or "").strip().rstrip("/").lower()
@@ -914,6 +924,7 @@ class GeminiNativeClient:
             thinking_config=thinking_config,
         )
 
+        model = bare_gemini_model_id(model)
         if stream:
             return self._stream_completion(model=model, request=request, timeout=timeout)