mssqlserver: guard Connect against concurrent calls

[mmatczuk]

The benthos framework can call Connect() concurrently or re-enter it
before the background goroutine has stopped. Without a guard, two
goroutines share the same publisher and checkpoint tracker, corrupting
state and causing the streaming phase to hang indefinitely.

Add a mutex and a HasStoppedChan check so that a second Connect() is
a no-op while the previous goroutine is still running.

Fixes CON-389

[claude]

Commits
LGTM

Review
Small, focused fix that adds a mutex and HasStoppedChan guard to Connect() to prevent concurrent/re-entrant calls from corrupting shared state. The mutex is correctly scoped inside the method (per mutex encapsulation rules), and the stopSig check correctly distinguishes "goroutine still active" from "ready to reconnect."

LGTM