chore: Lazy load Vertex AI dependencies in ADK modules

This is about 35% decrease. This change refactors several ADK modules to import `vertexai` and its submodules only when they are first used, rather than at the top of the file. This improves module load times by avoiding unnecessary imports of large dependencies. Imports are also placed within `if TYPE_CHECKING:` blocks where appropriate.

Co-authored-by: Liang Wu <wuliang@google.com>
PiperOrigin-RevId: 829017293

src/google/adk/sessions/vertex_ai_session_service.py

@@ -20,16 +20,14 @@ import logging
 import re
 from typing import Any
 from typing import Optional
+from typing import TYPE_CHECKING
 from typing import Union
 
 from google.genai import types
-from google.genai.errors import ClientError
-from tenacity import retry
-from tenacity import retry_if_result
-from tenacity import stop_after_attempt
-from tenacity import wait_exponential
 from typing_extensions import override
-import vertexai
+
+if TYPE_CHECKING:
+  import vertexai
 
 from . import _session_util
 from ..events.event import Event
@@ -326,6 +324,8 @@ class VertexAiSessionService(BaseSessionService):
     Returns:
       An API client for the given project and location or express mode api key.
     """
+    import vertexai
+
     return vertexai.Client(
         project=self._project,
         location=self._location,