fix(sessions): truncate error_message in v0 StorageEvent to guard against VARCHAR(255) overflow

Databases created with earlier ADK versions retain error_message as
VARCHAR(255) because create_all() is additive-only and never ALTERs
existing columns. When a model returns a MALFORMED_FUNCTION_CALL error
with a large JSON payload, the unbounded error_message silently exceeds
that limit and raises StringDataRightTruncationError, terminating the
SSE stream with no fallback.

The fix adds a write-path guard in StorageEvent.from_event(): if the
message exceeds 255 characters it is clipped to 241 chars and suffixed
with "...[truncated]", keeping the total within the legacy column width.
Messages within the limit are passed through unchanged. The v1 schema is
unaffected (error_message is stored inside the event_data JSONB column).

The permanent fix remains running:
  ALTER TABLE events ALTER COLUMN error_message TYPE TEXT;
or migrating to the v1 schema via `adk migrate session`.

Fixes #4993

Author: STHITAPRAJNAS

src/google/adk/sessions/schemas/v0.py

@@ -61,6 +61,13 @@
 from .shared import DynamicJSON
 from .shared import PreciseTimestamp
 
+# Legacy v0 databases may retain error_message as VARCHAR(255) from earlier
+# schema versions. create_all() is additive only, so existing columns are never
+# altered. Truncate on write to avoid StringDataRightTruncationError on those
+# databases.
+_LEGACY_ERROR_MSG_MAX_LEN = 255
+_TRUNCATION_SUFFIX = "...[truncated]"
+
 
 class DynamicPickleType(TypeDecorator):
   """Represents a type that can be pickled."""
@@ -298,7 +305,15 @@ def from_event(cls, session: Session, event: Event) -> StorageEvent:
         partial=event.partial,
         turn_complete=event.turn_complete,
         error_code=event.error_code,
-        error_message=event.error_message,
+        error_message=(
+            event.error_message[
+                : _LEGACY_ERROR_MSG_MAX_LEN - len(_TRUNCATION_SUFFIX)
+            ]
+            + _TRUNCATION_SUFFIX
+            if event.error_message
+            and len(event.error_message) > _LEGACY_ERROR_MSG_MAX_LEN
+            else event.error_message
+        ),
         interrupted=event.interrupted,
     )
     if event.content:

tests/unittests/sessions/test_v0_storage_event.py

@@ -15,9 +15,13 @@
 from datetime import datetime
 from datetime import timezone
 
+from google.adk.events.event import Event
 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
 
 
@@ -48,3 +52,37 @@ def test_storage_event_v0_to_event_rehydrates_compaction_model():
   assert isinstance(event.actions.compaction, EventCompaction)
   assert event.actions.compaction.start_timestamp == 1.0
   assert event.actions.compaction.end_timestamp == 2.0
+
+
+def test_from_event_truncates_error_message_exceeding_varchar255():
+  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,
+      error_code="MALFORMED_FUNCTION_CALL",
+      error_message="x" * 300,
+  )
+
+  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)
+
+
+def test_from_event_preserves_short_error_message():
+  session = Session(app_name="app", user_id="user", id="session_id")
+  short_message = "Malformed function call"
+  event = Event(
+      id="event_id",
+      invocation_id="invocation_id",
+      author="author",
+      timestamp=1.0,
+      error_code="MALFORMED_FUNCTION_CALL",
+      error_message=short_message,
+  )
+
+  storage_event = StorageEvent.from_event(session, event)
+
+  assert storage_event.error_message == short_message