feat: Expose Gemini RetryOptions to client

google.genai SDK has introduced a new retry_options. This change exposes this configuration
to ADK users

Usage:

```python
root_agent = Agent(
    model=Gemini(
        model='gemini-2.0-flash',
        retry_options=types.HttpRetryOptions(
	    initial_delay=1,
	    attempts=2
	    # ... Retry options from google.genai
	),
	# ...
    ),
```
PiperOrigin-RevId: 786472564

src/google/adk/models/google_llm.py

@@ -22,6 +22,7 @@ import os
 import sys
 from typing import AsyncGenerator
 from typing import cast
+from typing import Optional
 from typing import TYPE_CHECKING
 from typing import Union
 
@@ -57,6 +58,23 @@ class Gemini(BaseLlm):
 
   model: str = 'gemini-1.5-flash'
 
+  retry_options: Optional[types.HttpRetryOptions] = None
+  """Allow Gemini to retry failed responses.
+
+  Sample:
+  ```python
+  from google.genai import types
+
+  # ...
+
+  agent = Agent(
+    model=Gemini(
+      retry_options=types.HttpRetryOptions(initial_delay=1, attempts=2),
+    )
+  )
+  ```
+  """
+
   @staticmethod
   @override
   def supported_models() -> list[str]:
@@ -191,7 +209,10 @@ class Gemini(BaseLlm):
       The api client.
     """
     return Client(
-        http_options=types.HttpOptions(headers=self._tracking_headers)
+        http_options=types.HttpOptions(
+            headers=self._tracking_headers,
+            retry_options=self.retry_options,
+        )
     )
 
   @cached_property