fix(sessions): support MySQL and MariaDB schema creation

[linhongyu510]

This pull request fixes automatic SQLAlchemy session table creation on MySQL and MariaDB. Both dialects reject an unbounded String when compiling indexed VARCHAR columns, so SQLAlchemySession(create_tables=True) currently fails before issuing any DDL.

Summary

• Use a dialect-specific VARCHAR(190) for both session_id columns on MySQL and MariaDB.
• Preserve the existing unbounded VARCHAR type on PostgreSQL and SQLite.
• Keep the parent primary-key and child foreign-key column types identical.
• Use 190 characters so the (session_id, created_at) index remains within the traditional 767-byte InnoDB key limit under utf8mb4.
• Add offline MetaData.create_all() regression tests for MySQL and MariaDB covering both tables, ON DELETE CASCADE, and the full composite index.

Test plan

• uv run --extra sqlalchemy pytest -q tests/extensions/memory/test_sqlalchemy_session.py (46 passed)
• .agents/skills/code-change-verification/scripts/run.sh
  • format: passed
  • lint: passed
  • typecheck: passed
  • tests: passed

Issue number

Closes #4740

Checks

• I've added new tests, if relevant
• I've run .agents/skills/code-change-verification/scripts/run.sh
• I've confirmed all verification steps pass
• If using Codex, I've run /review before submitting this PR

[linhongyu510]

The Tests workflow is currently waiting with action_required on this first-time contribution. The branch is a single focused commit, is not behind main, and the changed source/test files pass Ruff and formatting checks locally. Could a maintainer approve the workflow run when convenient?

[linhongyu510]

Follow-up live-database verification completed against a fresh MySQL 8.0.46 instance using SQLAlchemy 2.0.43 and aiomysql 0.3.2.

I exercised the actual SQLAlchemySession.from_url(..., create_tables=True) path, then:

• inserted one user item and one assistant item,
• read both items back through get_items(),
• queried information_schema.COLUMNS and confirmed session_id is VARCHAR(190) in both agent_sessions and agent_messages,
• queried information_schema.STATISTICS and confirmed idx_agent_messages_session_time contains (session_id, created_at) in that order, and
• cleared the session and confirmed get_items() returned an empty list.

The run completed with MYSQL_E2E_OK. The database was an ephemeral local container and was removed after the check. No additional code changes were needed.