Fix client_prefetch_threads=0 normalization - #2843

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1. What GitHub issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes SNOW-3324331: client_prefetch_threads validation can bypass the lower bound due to ordering of casting and bounds checks #2841

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am modifying authorization mechanisms
   • I am adding new credentials
   • I am modifying OCSP code
   • I am adding a new dependency

3. Please describe how your code solves the related issue.

This change makes the existing defensive validation path for client_prefetch_threads internally consistent when a non-positive value reaches it.

In the sync path, _validate_client_prefetch_threads() previously read through the defaulting property, which meant a raw value of 0 could resolve to DEFAULT_CLIENT_PREFETCH_THREADS instead of following the existing lower-bound correction. This PR updates the validator to normalize from the raw backing value, preserve None as the default case, and then apply the existing bounds logic before storing the final value.

I also included corresponding async updates so sync and async behavior stay aligned. In particular, async now validates direct assignment through the setter and continues to validate explicitly provided client_prefetch_threads values during connection setup.

The PR adds targeted sync and async unit tests covering:

• connection construction with client_prefetch_threads=0
• direct assignment of client_prefetch_threads=0

4. (Optional) PR for stored-proc connector:
   N/A