fix(deployable): pin EnsureUploaded lock connection

[theophanemayaud]

Context

EnsureUploaded acquires a session-owned SQL Server application lock through *sql.DB and later releases it through another pool operation. database/sql can execute those calls on different physical sessions, causing SQL Server error 1223 (lock ... is not currently held) and leaving the original session responsible for the lock.

Changes

• pin one *sql.Conn across lock acquisition, schema existence checking, upload, and lock release
• reuse the pinned connection while retaining upload impersonation and transaction boundaries
• release the lock with a non-cancelled cleanup context and return release failures instead of discarding them
• add a SQL Server integration test that verifies schema creation runs on the session owning the application lock

The lock mode, timeout, schema/cache behavior, and public API remain unchanged.