move ai session cleanup after migration

[pjain1]

After #9500 rill start is throwing error like because the cleanup starts in open method before migration has run which creates these tables.

ERROR    sqlite: failed to delete expired AI sessions    {"error": "failed to query expired AI sessions: SQL logic error: no such table: ai_sessions (1)"}

Although its harmless but still move clean up after catalog migration has run.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!