perf(import): lazy-load heavy dependencies

END_PUBLIC
https://docs.google.com/document/d/1EpQaKqlX_OT4GrpbpqCLjtyF1pBcCTyu0sjXa8zWk8c/edit?usp=sharing

Deeply nested imports in new features (such as `events` pulling in `auth`, which eagerly imports `FastAPI`; and `functions` eagerly importing `OpenTelemetry`) silently added over 400ms to the baseline import time at HEAD, negating recent performance gains.

This CL prevents heavy external frameworks from accidentally polluting the top-level `import google.adk` path by:
- Deferring FastAPI by converting `auth/__init__.py` to a lazy-loading registry via `__getattr__`.
- Moving `AuthConfig` imports behind `if TYPE_CHECKING:` in `event_actions.py`, `functions.py`, and `base_credential_service.py`.
- Deferring `OpenTelemetry` by moving `from ..telemetry import _instrumentation` inside `run_async`/`run_live` and moving `from opentelemetry import trace` behind `if TYPE_CHECKING:`.
- Adding an `import_tree_audit.py` script and a formal RFC for import time bloat prevention.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 952421569

Author: GWeale

src/google/adk/events/event_actions.py

@@ -18,6 +18,7 @@
 from typing import Any
 from typing import cast
 from typing import Optional
+from typing import TYPE_CHECKING
 from typing import Union
 
 from google.genai.types import Content
@@ -26,13 +27,18 @@
 from pydantic import ConfigDict
 from pydantic import Field
 from pydantic import field_serializer
+from pydantic import field_validator
 from pydantic import SerializerFunctionWrapHandler
 from pydantic_core import to_jsonable_python
 
-from ..auth.auth_tool import AuthConfig
 from ..tools.tool_confirmation import ToolConfirmation
 from .ui_widget import UiWidget
 
+if TYPE_CHECKING:
+  from ..auth.auth_tool import AuthConfig
+else:
+  AuthConfig = Any
+
 logger = logging.getLogger('google_adk.' + __name__)
 
 
@@ -131,6 +137,18 @@ def _serialize_state_delta(
   - Values: The requested auth config.
   """
 
+  @field_validator('requested_auth_configs', mode='after')
+  @classmethod
+  def _parse_auth_configs(cls, v: dict[str, Any]) -> dict[str, Any]:
+    if not v:
+      return v
+    from ..auth.auth_tool import AuthConfig
+
+    return {
+        k: AuthConfig.model_validate(val) if isinstance(val, dict) else val
+        for k, val in v.items()
+    }
+
   requested_tool_confirmations: dict[str, ToolConfirmation] = Field(
       default_factory=dict
   )