Ensure MySQL kv_cache.v can store large payloads and perform one-time schema check

[zhenghaoz]

Motivation

• Prevent truncation of large cached payloads by ensuring the kv_cache.v column can hold large text in MySQL.
• Run the schema adjustment in a thread-safe, one-time manner to avoid repeated DDL and race conditions.

Description

• Imported Lock and added module-level _kv_cache_schema_checked and _kv_cache_schema_lock flags to coordinate a one-time check.
• Added ensure_kv_cache_schema(engine) which alters the kv_cache table to set column v to LONGTEXT when engine.dialect.name == "mysql" and marks the schema check complete.
• Invoke ensure_kv_cache_schema(engine) at the start of get_cached and save_cache so the schema is validated/updated before cache reads/writes.

Testing

• Ran the existing unit test suite including cache-related tests, and they passed.
• Ran integration verification against a MySQL instance to confirm the v column is converted to LONGTEXT and caching round-trips succeed, and the tests passed.

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Codex Task

[Copilot]

Pull request overview

Updates the caching layer to prevent truncation of large cached payloads in MySQL by ensuring kv_cache.v can store large text, and attempts to make the schema adjustment run only once per process.

Changes:

• Added a module-level lock/flag and ensure_kv_cache_schema(engine) to adjust kv_cache.v to LONGTEXT on MySQL.
• Invoked the schema ensure step before cache reads/writes in get_cached() and save_cache().

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