fix(sessions): use LONGTEXT for MySQL message storage

[rioyu123]

Summary

This pull request changes the message_data column to LONGTEXT for newly created MySQL and MariaDB message tables. Other dialects retain TEXT; serialization, session identifiers, transactions, and public APIs are unchanged. Existing tables are not migrated.

MySQL-family TEXT storage is too small for some serialized session items. In a live example, Chinese/emoji content serialized to 1,200,028 bytes: strict mode rejected the TEXT insert with error 1406, while non-strict mode logged truncation and stored 65,535 bytes of invalid JSON that the SDK could not return as the original item. A LONGTEXT column preserved the full item through SDK add/get/pop in both modes.

Draft dependency: current main cannot automatically create these MySQL/MariaDB tables because the session_id columns use an unbounded String. #4741, an open PR from another contributor, proposes fixing that separate problem. This PR contains none of its changes and does not independently restore automatic table creation. It is open as a draft to discuss the column-capacity change while the schema-creation work is reviewed. Users who already manage their own LONGTEXT schema do not need this patch.

Before marking this ready, I will refresh against the accepted schema-creation baseline, verify the combined automatic-create and large-item path, and finish the final review and verification gates.

Test plan

• tests/extensions/memory/test_sqlalchemy_session.py: 47 passed, including four new column-compilation cases for MySQL, MariaDB, PostgreSQL, and SQLite.
• Ruff check and format check passed for both changed files; git diff --check passed.
• Real MySQL 8.0.46 and MariaDB 11.8.9, using caller-managed tables and SQLAlchemySession(create_tables=False): small-item controls passed; strict TEXT raised error 1406; non-strict TEXT produced truncated, invalid JSON; LONGTEXT preserved the complete 1,200,028-byte serialized value and exact SDK get/pop results in both modes.
• Offline complete-table compilation: current main plus this change still fails on unbounded session_id; fix(sessions): support MySQL and MariaDB schema creation #4741's schema plus the proposed column type compiles for both MySQL and MariaDB. This was a disposable metadata composition check, not a combined-source live automatic-creation test.
• The full repository verification stack and final schema-change review are still pending for this early draft. No full-stack pass is claimed.

Issue number

N/A. Related schema-creation work: #4741.

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