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