fix(sessions): emit logger.warning with full message before truncating error_message

Truncation is a lossy operation. Without surfacing the original content,
users hitting this path on a legacy VARCHAR(255) schema had no way to
recover the full error text without attaching a debugger.

Extract the truncation logic into a pre-flight block before the
StorageEvent constructor, emit a WARNING with the event id, the length
limit, and the full original message, then pass the already-truncated
value into the constructor. This makes the information available at
WARNING level in any standard logging setup.

Also adds import logging and module-level logger following the
google_adk.<module> convention already used in the sessions package.

Test updated to assert the warning is emitted and contains the original
full message.

Author: STHITAPRAJNAS

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

@@ -29,10 +29,13 @@
 from datetime import datetime
 from datetime import timezone
 import json
+import logging
 import pickle
 from typing import Any
 from typing import Optional
 
+logger = logging.getLogger("google_adk." + __name__)
+
 from google.adk.platform import uuid as platform_uuid
 from google.genai import types
 from sqlalchemy import Boolean
@@ -291,6 +294,19 @@ def long_running_tool_ids(self, value: set[str]):
 
   @classmethod
   def from_event(cls, session: Session, event: Event) -> StorageEvent:
+    error_message = event.error_message
+    if error_message and len(error_message) > _LEGACY_ERROR_MSG_MAX_LEN:
+      logger.warning(
+          "error_message for event %s exceeds %d characters and will be"
+          " truncated. Full message: %s",
+          event.id,
+          _LEGACY_ERROR_MSG_MAX_LEN,
+          error_message,
+      )
+      error_message = (
+          error_message[: _LEGACY_ERROR_MSG_MAX_LEN - len(_TRUNCATION_SUFFIX)]
+          + _TRUNCATION_SUFFIX
+      )
     storage_event = StorageEvent(
         id=event.id,
         invocation_id=event.invocation_id,
@@ -305,15 +321,7 @@ 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[
-                : _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
-        ),
+        error_message=error_message,
         interrupted=event.interrupted,
     )
     if event.content:

tests/unittests/sessions/test_v0_storage_event.py

@@ -52,7 +52,9 @@ def test_storage_event_v0_to_event_rehydrates_compaction_model():
   assert event.actions.compaction.end_timestamp == 2.0
 
 
-def test_from_event_truncates_error_message_exceeding_varchar255():
+def test_from_event_truncates_error_message_exceeding_varchar255(caplog):
+  import logging
+
   session = Session(app_name="app", user_id="user", id="session_id")
   event = Event(
       id="event_id",
@@ -63,9 +65,12 @@ def test_from_event_truncates_error_message_exceeding_varchar255():
       error_message="x" * 300,
   )
 
-  storage_event = StorageEvent.from_event(session, event)
+  with caplog.at_level(logging.WARNING):
+    storage_event = StorageEvent.from_event(session, event)
 
   assert storage_event.error_message == "x" * 241 + "...[truncated]"
+  assert any("truncated" in r.message for r in caplog.records)
+  assert any("x" * 300 in r.message for r in caplog.records)
 
 
 def test_from_event_preserves_short_error_message():