fix: consistent UTC timestamp handling for PostgreSQL in DatabaseSessionService

Fixes two timestamp inconsistencies that cause stale session detection
failures when using PostgreSQL with asyncpg:

1. append_event() now strips timezone info for PostgreSQL (matching
   create_session() behavior). Previously, PostgreSQL got a local-time
   naive datetime via datetime.fromtimestamp(), while create_session()
   stored a UTC-based naive datetime. This mismatch caused
   get_update_timestamp() to return incorrect values when the server
   timezone differs from UTC.

2. get_update_timestamp() now uses tzinfo-awareness to decide whether
   to interpret a naive datetime as UTC, rather than checking only for
   SQLite. This correctly handles both SQLite and PostgreSQL naive
   datetimes without requiring callers to pass dialect flags.

Fixes #4366
Related: #1848

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: anmolg1997

src/google/adk/sessions/database_session_service.py

@@ -683,12 +683,12 @@ async def append_event(self, session: Session, event: Event) -> Event:
               storage_session.state | state_deltas["session"]
           )
 
-        if is_sqlite:
-          update_time = datetime.fromtimestamp(
-              event.timestamp, timezone.utc
-          ).replace(tzinfo=None)
-        else:
-          update_time = datetime.fromtimestamp(event.timestamp)
+        # Convert event timestamp to a UTC datetime.  SQLite and PostgreSQL
+        # use TIMESTAMP WITHOUT TIME ZONE, so the tzinfo must be stripped to
+        # avoid asyncpg DataError / SQLite comparison issues.
+        update_time = datetime.fromtimestamp(event.timestamp, timezone.utc)
+        if is_sqlite or self.db_engine.dialect.name == _POSTGRESQL_DIALECT:
+          update_time = update_time.replace(tzinfo=None)
         storage_session.update_time = update_time
         sql_session.add(schema.StorageEvent.from_event(session, event))
 

src/google/adk/sessions/schemas/v1.py

@@ -119,12 +119,15 @@ def update_timestamp_tz(self) -> float:
     )
     return self.get_update_timestamp(is_sqlite=is_sqlite)
 
-  def get_update_timestamp(self, is_sqlite: bool) -> float:
-    """Returns the time zone aware update timestamp."""
-    if is_sqlite:
-      # SQLite does not support timezone. SQLAlchemy returns a naive datetime
-      # object without timezone information. We need to convert it to UTC
-      # manually.
+  def get_update_timestamp(self, is_sqlite: bool = False) -> float:
+    """Returns the update timestamp as a POSIX float.
+
+    Naive datetimes (returned by SQLite and PostgreSQL which use
+    ``TIMESTAMP WITHOUT TIME ZONE``) are interpreted as UTC.  Timezone-aware
+    datetimes (MySQL, MariaDB) are used directly.
+    """
+    if self.update_time.tzinfo is None:
+      # Naive datetimes from SQLite / PostgreSQL are stored as UTC.
       return self.update_time.replace(tzinfo=timezone.utc).timestamp()
     return self.update_time.timestamp()