fix: identify account facts HTTP client explicitly

[Pigbibi]

Summary

• Add the explicit QSL client User-Agent to the isolated facts POST and readiness GET.
• Keep the endpoint, authentication, validation, acknowledgement rules, timeout, no-redirect policy, and single-request behavior unchanged.
• Preserve the frozen production caller/application and environment protection.

Evidence

A bounded unauthenticated GET from the existing execution node using the default Python client returned HTTP 403. With only the explicit client header changed, the same endpoint returned the expected HTTP 401 authentication response. This demonstrates a client path difference; actual protected publishing remains to be verified.

Validation

129 targeted synthetic tests passed. Ruff and git diff checks passed. Tests assert the exact endpoint, method, User-Agent, timeout, no redirect handler, and one request.