[imran-siddique]

Summary

• Adds expected_measurement field to AttestationConfig (optional, defaults to None)
• SEVSNPProvider.__init__ accepts the value and stores it as _expected_measurement
• On each get_attestation_report call, if configured, computes sha384 of the raw firmware measurement bytes and uses hmac.compare_digest for constant-time comparison — raises RuntimeError("measurement mismatch") on failure
• detect_provider / _get_provider_impl now thread the config through so SEVSNPProvider receives expected_measurement at construction

Closes #172

Test plan

• test_expected_measurement_loaded_from_config — field parsed from YAML
• test_expected_measurement_defaults_to_none — omitting field leaves it None
• test_expected_measurement_non_string_rejected — non-string value raises ConfigError
• test_sevsnp_no_expected_measurement_skips_check — None value skips comparison
• test_sevsnp_stores_expected_measurement — value stored on instance
• test_sevsnp_rejects_mismatched_expected_measurement — mismatch raises RuntimeError
• test_sevsnp_accepts_matching_expected_measurement — match returns report cleanly
• test_detect_provider_passes_expected_measurement_to_snp — wired end-to-end