test(sessions): tighten v0 error_message truncation assertions per review

- Replace <= with == in the truncation test: the guard is deterministic
  so the result should be exactly 255 chars, not merely at most 255
- Assert the concrete expected string ("x" * 241 + "...[truncated]")
  rather than importing private module symbols, reducing coupling to
  internal implementation details
- Add test_from_event_with_none_error_message to cover the falsy branch
  of the truncation condition and document that None passes through
  unchanged

Author: STHITAPRAJNAS

tests/unittests/sessions/test_v0_storage_event.py

@@ -19,8 +19,6 @@
 from google.adk.events.event_actions import EventActions
 from google.adk.events.event_actions import EventCompaction
 from google.adk.sessions.schemas.v0 import StorageEvent
-from google.adk.sessions.schemas.v0 import _LEGACY_ERROR_MSG_MAX_LEN
-from google.adk.sessions.schemas.v0 import _TRUNCATION_SUFFIX
 from google.adk.sessions.session import Session
 from google.genai import types
 
@@ -67,8 +65,7 @@ def test_from_event_truncates_error_message_exceeding_varchar255():
 
   storage_event = StorageEvent.from_event(session, event)
 
-  assert len(storage_event.error_message) <= _LEGACY_ERROR_MSG_MAX_LEN
-  assert storage_event.error_message.endswith(_TRUNCATION_SUFFIX)
+  assert storage_event.error_message == "x" * 241 + "...[truncated]"
 
 
 def test_from_event_preserves_short_error_message():
@@ -86,3 +83,17 @@ def test_from_event_preserves_short_error_message():
   storage_event = StorageEvent.from_event(session, event)
 
   assert storage_event.error_message == short_message
+
+
+def test_from_event_with_none_error_message():
+  session = Session(app_name="app", user_id="user", id="session_id")
+  event = Event(
+      id="event_id",
+      invocation_id="invocation_id",
+      author="author",
+      timestamp=1.0,
+  )
+
+  storage_event = StorageEvent.from_event(session, event)
+
+  assert storage_event.error_message is None