Reset ThrottleHTTPStatusCode when throttle-http URL is cleared

[dnovitski]

Problem

When an operator changes or clears the throttle-http URL via interactive command, the HTTP polling stops (or switches endpoints) but the stale ThrottleHTTPStatusCode is never reset. If the last HTTP response was non-200 (e.g. 503), shouldThrottle() keeps reading that code and the migration stays throttled forever with no way to unthrottle it.

This also affects URL changes: switching from one endpoint to another carries over the old endpoint's status code, causing incorrect throttling until the new endpoint is polled.

Reproduction scenario

1. Migration runs with --throttle-http=http://service/check
2. Service returns 503 → ThrottleHTTPStatusCode stored as 503
3. shouldThrottle() sees 503 != 0 && 503 != 200 → migration throttles ✅
4. Operator clears URL: throttle-http= (to disable HTTP throttling)
5. collectHTTPThrottleResponse sees empty URL → stops polling
6. Bug: ThrottleHTTPStatusCode still holds 503
7. shouldThrottle() keeps reading 503 → migration stays throttled forever ❌

Fix

Reset ThrottleHTTPStatusCode to 0 in SetThrottleHTTP() whenever the URL changes. 0 passes the statusCode != 0 guard in shouldThrottle(), properly disabling HTTP throttling. When a new URL is set, the next poll will populate the correct status code from the new endpoint.

Tests

• TestSetThrottleHTTP_ClearsStatusCode: status code resets to 0 when URL is cleared (fails without fix: expected 0, got 503)
• TestSetThrottleHTTP_PreservesStatusCodeWhenURLSet: status code also resets when switching to a new URL (stale code from old endpoint is meaningless)