ci: restore Dashboard stack validation contracts

[seonghobae]

Summary

• update the four _request test doubles that exercise post_json to accept its response-control-header sink
• narrowly suppress Semgrep rule python37-compatibility-importlib2 on the standard-library import because pyproject.toml requires Python 3.12 or newer
• restore the exact Dashboard stack gates without changing production behavior

Verification

• uv run --frozen --extra dev python -m pytest -q tests/test_contextual_orchestrator_vision.py tests/test_observability.py tests/test_ontology.py (44 passed)
• git diff --check

Stacked on #640 exact head fa9f0aeb5225035264ebc0579c127d1f283c0b17; merge this repair into that branch before revalidating #640.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.