fix(sessions): correct timezone handling for PostgreSQL in DatabaseSessionService

[zouyi100]

Problem:
create_session already stored UTC naive datetimes for both SQLite and
PostgreSQL, but the corresponding read path (get_update_timestamp) and
write path in append_event only handled SQLite. On non-UTC hosts this
causes session.last_update_time to be off by the local timezone offset
after create_session, and potentially triggers false stale-session errors
after the first append_event call on PostgreSQL.

Three specific inconsistencies:

1. get_update_timestamp only added UTC tzinfo for SQLite, not PostgreSQL.
2. append_event wrote local-time naive update_time for PostgreSQL, while
   create_session wrote UTC naive.
3. get_session / list_sessions did not propagate is_postgresql to
   to_session.

Solution:
Extend the existing SQLite UTC-naive convention to PostgreSQL consistently
across all read and write paths.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

pytest tests/unittests/sessions/ --deselect
"tests/unittests/sessions/test_session_service.py::test_get_session_with_config[SessionServiceType.DATABASE]"
169 passed (the deselected test is a pre-existing Windows-only OSError

unrelated to this change)

Manual End-to-End (E2E) Tests:
Tested with SQLite (:memory:) and verified session.last_update_time
returns correct UTC-based POSIX timestamps after create_session and
append_event.

#5354

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[rohityan]

Hi @zouyi100 , Thank you for your contribution! We appreciate you taking the time to submit this pull request.
For code to be cleaner It is better to abstract the timestamp creation into a single helper function that way we can avoid passing the is_postgresql flag through multiple function signatures.
Also could you also add a new test case that handles the fix.

[zouyi100]

Hi @rohityan
Done and please review the code. I have also test again on my PostgreSQL project and working well.