fix: strip tzinfo from datetime for MySQL in create_session

Merge google#5084

### Link to Issue or Description of Change

Fixes google#5085

When using `DatabaseSessionService` with MySQL, the very first `append_event()` after `create_session()` always fails with:

```
ValueError: The session has been modified in storage since it was loaded.
Please reload the session before appending more events.
```

**Root cause:** `create_session()` produces a timezone-aware `_storage_update_marker` (with `+00:00`), but MySQL `DATETIME(fsp=6)` doesn't store timezone info ([MySQL docs](https://dev.mysql.com/doc/refman/8.0/en/datetime.html)). When `append_event()` reads `update_time` back, it gets a naive datetime — different marker string, strict comparison fails.

The code already strips `tzinfo` for SQLite and PostgreSQL but MySQL was missed:

```python
# Before (line 464):
if is_sqlite or is_postgresql:        # MySQL not included
    now = now.replace(tzinfo=None)

# After:
is_mysql = self.db_engine.dialect.name == _MYSQL_DIALECT
if is_sqlite or is_postgresql or is_mysql:
    now = now.replace(tzinfo=None)
```

### Testing Plan

**Unit Tests:**

- [x] I have added or updated unit tests for my change.
- [x] All unit tests pass locally.

Added `mysql` to the existing parametrized dialect test. Removed the test that incorrectly asserted MySQL preserves timezone.

**Manual End-to-End (E2E) Tests:**

Verified marker values with Testcontainers MySQL 8.0:

| | create_session marker | get_session marker | Match? |
|---|---|---|---|
| Before fix | `...T14:39:46.014977+00:00` | `...T14:39:46.014977` | No |
| After fix | `...T14:44:49.388273` | `...T14:44:49.388273` | Yes |

Confirmed that `append_event()` works immediately after `create_session()` without needing a re-fetch workaround.

### Checklist

- [x] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [x] I have performed a self-review of my own code.
- [x] I have commented my code, particularly in hard-to-understand areas.
- [x] I have added tests that prove my fix is effective or that my feature works.
- [x] New and existing unit tests pass locally with my changes.
- [x] I have manually tested my changes end-to-end.
- [x] Any dependent changes have been merged and published in downstream modules.

Co-authored-by: Haran Rajkumar <haranrk@google.com>
COPYBARA_INTEGRATE_REVIEW=google#5084 from andreikravchenko-oviva:fix/mysql-stale-session-marker 30bccf1
PiperOrigin-RevId: 934446269

Author: 2 people

src/google/adk/sessions/database_session_service.py

@@ -531,7 +531,8 @@ async def create_session(
       now = datetime.fromtimestamp(platform_time.get_time(), tz=timezone.utc)
       is_sqlite = self.db_engine.dialect.name == _SQLITE_DIALECT
       is_postgresql = self.db_engine.dialect.name == _POSTGRESQL_DIALECT
-      if is_sqlite or is_postgresql:
+      is_mysql = self.db_engine.dialect.name == _MYSQL_DIALECT
+      if is_sqlite or is_postgresql or is_mysql:
         now = now.replace(tzinfo=None)
 
       storage_session = schema.StorageSession(

tests/unittests/sessions/test_session_service.py

@@ -106,47 +106,31 @@ def fake_create_async_engine(_db_url: str, **kwargs):
   assert captured_kwargs.get('pool_pre_ping') is True
 
 
-@pytest.mark.parametrize('dialect_name', ['sqlite', 'postgresql'])
+@pytest.mark.parametrize('dialect_name', ['sqlite', 'postgresql', 'mysql'])
 def test_database_session_service_strips_timezone_for_dialect(dialect_name):
   """Verifies that timezone-aware datetimes are converted to naive datetimes
-  for SQLite and PostgreSQL to avoid 'can't subtract offset-naive and
-  offset-aware datetimes' errors.
+  for SQLite, PostgreSQL, and MySQL.
 
-  PostgreSQL's default TIMESTAMP type is WITHOUT TIME ZONE, which cannot
-  accept timezone-aware datetime objects when using asyncpg. SQLite also
-  requires naive datetimes.
+  These databases store DATETIME/TIMESTAMP WITHOUT TIME ZONE, so keeping
+  tzinfo on the Python datetime causes a mismatch between the marker
+  produced by create_session (with +00:00) and the marker read back by
+  get_session (without +00:00), triggering a false stale-writer error
+  on the first append_event after create_session.
   """
   # Simulate the logic in create_session
   is_sqlite = dialect_name == 'sqlite'
   is_postgres = dialect_name == 'postgresql'
+  is_mysql = dialect_name == 'mysql'
 
   now = datetime.now(timezone.utc)
   assert now.tzinfo is not None  # Starts with timezone
 
-  if is_sqlite or is_postgres:
+  if is_sqlite or is_postgres or is_mysql:
     now = now.replace(tzinfo=None)
 
-  # Both SQLite and PostgreSQL should have timezone stripped
   assert now.tzinfo is None
 
 
-def test_database_session_service_preserves_timezone_for_other_dialects():
-  """Verifies that timezone info is preserved for dialects that support it."""
-  # For dialects like MySQL with explicit timezone support, we don't strip
-  dialect_name = 'mysql'
-  is_sqlite = dialect_name == 'sqlite'
-  is_postgres = dialect_name == 'postgresql'
-
-  now = datetime.now(timezone.utc)
-  assert now.tzinfo is not None
-
-  if is_sqlite or is_postgres:
-    now = now.replace(tzinfo=None)
-
-  # MySQL should preserve timezone (if the column type supports it)
-  assert now.tzinfo is not None
-
-
 def test_database_session_service_respects_pool_pre_ping_override():
   captured_kwargs = {}