Function validateAndPingUrl

Validates a URL's syntax and tests whether the server is reachable using a simple HEAD request with file extension probe fallback.

Strategy:

1. HEAD → 2xx/3xx → reachable, no proxy needed.
2. HEAD → 4xx/5xx → try file extension probe. If valid → reachable. Otherwise → not reachable.
3. HEAD → CORS → try file extension probe through proxy. If valid → reachable + needsProxy. Otherwise → not reachable.
4. HEAD → network/timeout → server unreachable.

This function has no OGC/GetCapabilities knowledge. For OGC-aware validation, use validateAndPingUrl. The function never throws — all failures are returned as part of the result object.