fix(secops): ensure consistent string return types in security tools

[dandye]

Summary

Resolves #161:

• Enforces the -> str return type contract in get_threat_intel by ensuring non-string or nested dictionary/list responses from the Gemini model are serialized with json.dumps().
• Corrects a copy-paste error in do_update_security_alert exception handling ("retrieving" -> "updating").
• Updates integration test assertions in test_secops_mcp.py to match the str (JSON-encoded) return types of get_security_alert_by_id and do_update_security_alert.
• Adds hermetic unit test coverage:
  • server/secops/tests/test_threat_intel_unit.py: 7 tests covering GeminiResponse objects, block formats, string answers, nested dictionary answers, direct string responses, unexpected response formats, and error handling.
  • server/secops/tests/test_security_alerts_unit.py: 6 additional tests covering get_security_alerts empty/error paths, get_security_alert_by_id success/error paths, and do_update_security_alert success/error paths.

Fixes #161

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Verification

• Hermetic Unit Tests:
  Ran full test suite in server/secops:

  uv run --directory server/secops --extra test pytest tests/test_security_alerts_unit.py tests/test_threat_intel_unit.py tests/test_secops_tools_unit.py tests/test_secops_parser_management_unit.py

  Result: 38 passed in 1.45s

• Linter & Formatting:

  uvx ruff check tests/test_security_alerts_unit.py tests/test_threat_intel_unit.py
  uvx ruff format --check tests/test_security_alerts_unit.py tests/test_threat_intel_unit.py

  Result: All checks passed.