chore: Defer import of live, Client and _transformers in google.genai

This is targeting the Client import mostly, but also prevents future latency increase if the other modules in genai adds more 3p dependencies. This change will make ADK only import `live`, `Client` and `_transformers` just-in-time, therefore cutting down cold start latency.

Co-authored-by: Liang Wu <wuliang@google.com>
PiperOrigin-RevId: 831124329

src/google/adk/models/google_llm.py

@@ -27,7 +27,6 @@ from typing import Optional
 from typing import TYPE_CHECKING
 from typing import Union
 
-from google.genai import Client
 from google.genai import types
 from typing_extensions import override
 
@@ -41,6 +40,8 @@ from .gemini_llm_connection import GeminiLlmConnection
 from .llm_response import LlmResponse
 
 if TYPE_CHECKING:
+  from google.genai import Client
+
   from .llm_request import LlmRequest
 
 logger = logging.getLogger('google_adk.' + __name__)
@@ -200,6 +201,8 @@ class Gemini(BaseLlm):
     Returns:
       The api client.
     """
+    from google.genai import Client
+
     return Client(
         http_options=types.HttpOptions(
             headers=self._tracking_headers,
@@ -239,6 +242,8 @@ class Gemini(BaseLlm):
 
   @cached_property
   def _live_api_client(self) -> Client:
+    from google.genai import Client
+
     return Client(
         http_options=types.HttpOptions(
             headers=self._tracking_headers, api_version=self._live_api_version