/{driver,open_serialization_test}.go: serialize open calls

[coffeegoddd]

No description provided.

[Copilot]

Pull request overview

This PR introduces process-wide serialization for embedded engine initialization to avoid concurrent access to global mutable state inside go-mysql-server (GMS), and adds tests to validate both serialization and context-cancellation behavior.

Changes:

• Add a process-level semaphore around openSqlEngine to ensure only one engine open happens at a time.
• Respect ctx.Done() while waiting to acquire the semaphore and return ctx.Err() when cancelled.
• Add tests to verify concurrent opens are serialized and that blocked opens respect context deadlines.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
driver.go	Adds a process-level semaphore to serialize openSqlEngine calls and handles context cancellation while waiting.
open_serialization_test.go	Adds new tests covering open serialization across connectors and cancellation while blocked on the semaphore.

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[reltuk]

LGTM!