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.

[andreikravchenko-oviva]

@rohityan Hi
Could you please take a look at this PR?
I saw that you ran /gemini review for the duplicate one - #5209

[rohityan]

Hi @andreikravchenko-oviva ,Thank you for your contribution! We appreciate you taking the time to submit this pull request. Your PR has been received by the team and is currently under review. We will provide feedback as soon as we have an update to share.

[rohityan]

Hi @DeanChensj , can you please review this.