fix(mysql): prevent panic on NULL values and hanging connections with OceanBase

[Blankll]

Problem

Connecting to OceanBase (MySQL-compatible) has two critical issues:

1. Panic on NULL values in test_connection()

thread 'tokio-rt-worker' panicked at mysql_common/src/value/convert/mod.rs:122:23:
Could not retrieve `alloc::string::String`: Couldn't convert the value `Null` to a desired type

row.get::<String, _>().unwrap() uses FromValue for String which panic!s on Value::NULL. OceanBase returns NULL for DATABASE() when no database is selected, and potentially for USER() depending on configuration.

2. Connection hangs indefinitely

MySQLAdapter::connect() had no internal connection timeout. The tcp_connect_timeout API was removed in mysql_async v0.34 with no replacement, so pool.get_conn().await could hang forever on unresponsive hosts.

The tokio::time::timeout in connect_server only covers one code path — the "Test Connection" dialog path (server::test_connection) calls connect() directly with zero timeout protection.

Fix

1. Safe NULL handling in test_connection()

Replaced row.get::<String, _>().unwrap() with row.get_opt::<String, _>() which returns Option<Result<String, FromValueError>>. A match on Some(Ok(val)) gracefully resolves NULL to None instead of panicking.

2. Adapter-level connection timeout

Added tokio::time::timeout wrapping pool.get_conn() and conn.query_drop(\"SELECT 1\") inside connect(), using self.config.connect_timeout_secs. This applies to ALL call paths (both "Connect" and "Test Connection") since the timeout lives at the adapter level.

Testing

• ✅ All 268 existing unit tests pass
• ✅ All 26 integration tests pass
• ✅ No regressions in MySQL adapter tests
• ✅ cargo check clean