fix: strip tzinfo from datetime for MySQL in create_session

[andreikravchenko-oviva]

Link to Issue or Description of Change

Fixes #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). 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:

# 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:

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

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.