feat: update all clients to use api.unsandbox.com/keys/validate

Change key validation endpoint from portal to API across all clients:
- C: Update PORTAL_BASE to API_BASE in validate_api_key()
- Python (sync/async): Change PORTAL_BASE to API_BASE
- Perl: Change $PORTAL_BASE to $API_BASE
- Rust (sync/async): Change hardcoded URL to api.unsandbox.com

All other languages (Go, Java, Kotlin, C#, Ruby, PHP, JavaScript) already
use their makeRequest() function with API_BASE, so no changes needed.

The key subcommand already exists and continues to work with the new endpoint.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

clients/python/async/src/un_async.py

@@ -1635,7 +1635,7 @@ async def validate_keys(
     """
     public_key, secret_key = _resolve_credentials(public_key, secret_key)
 
-    url = f"{PORTAL_BASE}/keys/validate"
+    url = f"{API_BASE}/keys/validate"
     timestamp = int(time.time())
     body = ""