fix: close database handles after failed open

[marcus-pousette]

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Problem

SQLite can return a database handle even when sqlite3_open_v2 fails, and requires callers to close it. The wrapper currently tracks that failed handle, registers extension functions, and then throws before the caller can receive and release it.

See the sqlite3_open_v2 contract.

Change

• Preserve sqlite3_errmsg before cleanup.
• Best-effort close and untrack a handle returned by a failed open.
• Register extension functions only after a successful open.

The focused regression test forces SQLITE_CANTOPEN with a non-null handle and proves that the original error is reported, the handle is closed and untracked, and extension registration is skipped.

[rhashimoto]

Thanks for the PR! That's a good catch.

Please fill out the contribution checklist that I prepended to your top-level PR message before I merge.