[AUD-002] Azure assessment silently no-ops on null/empty input — returns 200 with no action taken

## Summary

**Severity:** P2 — Silent Failure
**Phase:** Phase 4 Remediation (API Contract Hardening)
**Audit Reference:** SPIN Agent Product Integrity Report, Phase 1

## Finding

The Azure assessment endpoint accepts null or empty input and returns HTTP 200 without performing any assessment work. The caller receives a success response with no indication that nothing happened.

## Impact

- Automated pipelines and integrations cannot detect a no-op assessment call
- Monitoring/ConMon workflows may log a 'completed' assessment that produced nothing
- No client-side visibility into why results are empty

## Acceptance Criteria

- [ ] Return HTTP 400 or 422 when Azure assessment input is null or missing required fields
- [ ] Error response includes a machine-readable error code and human-readable message
- [ ] Unit test: assert 400 returned on null input
- [ ] No regression on valid assessment calls

## Related

- #829 — AUD-001 findings not persisted
- Audit artifact: SPIN Agent Product Integrity Report (Phase 1)

Start at the Azure assessment endpoint and trace how null, empty, or missing-field input is handled. Add the requested validation and error response, then create or update a unit test asserting a 400 response while confirming valid assessment calls remain unaffected.

Written by the indexing model from the issue text.