test: materialize deterministic MCP corruption fixtures

[ProfRandom92]

Summary:

• Add a deterministic materializer that reads artifacts/mcp_trace_corruption_manifest.json and materializes corruption fixtures for exactly three operators: DROP_APPROVAL_GATE, REMOVE_DEPENDENCY_EDGE, and TRUNCATE_RECOVERY_PATH while preserving the original fixture schema (trace.json, dependency_graph.json, state.json).
• Materialize the committed corpus under fixtures/mcp_trace_replay_corruptions/<source_fixture_slug>/<operator_slug>/ for the three existing MCP families (degraded, mild, moderate).
• Add focused tests that assert directory presence, operator scope, file-shape preservation, single-surface mutations, deterministic regeneration, absence of environment/time tokens, and manifest/failure-label alignment, and leave artifacts/evidence_index.json unchanged.

Changed files:

• scripts/materialize_mcp_trace_corruptions.py (new deterministic materializer).
• tests/test_mcp_trace_corruption_materialization.py (new focused materialization tests).
• fixtures/mcp_trace_replay_corruptions/** (new materialized fixture directories for the three selected operators across three source families).

Testing:

• Ran python scripts/materialize_mcp_trace_corruptions.py successfully to produce fixtures/mcp_trace_replay_corruptions/.
• Ran pytest -q tests/test_mcp_trace_corruption_materialization.py which passed (8 passed).
• Ran npm run check which completed successfully with full test-suite green (299 passed).

Risks:

• Low risk because changes are limited to fixture generation and tests and do not modify validators, workflows, package files, or failure labels.
• Materializer enforces manifest applicability and will raise deterministic repo-relative errors if source fixtures are missing or malformed.

Next:

• Materialize remaining operators (SWAP_TOOL_ORDER, INSERT_UNVALIDATED_ACTION, COLLAPSE_CAPABILITY_BOUNDARY) in follow-up PRs and optionally wire these fixtures into validator-level checks.

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Codex Task

[gemini-code-assist]

Code Review

This pull request introduces a materialization script and a corresponding test suite to generate deterministic MCP trace corruption fixtures from a manifest, alongside a comprehensive set of JSON fixtures for various failure scenarios. The reviewer feedback consistently identifies a violation of repository rules regarding JSON processing: null values for expected list fields should be treated as empty lists rather than immediately raising a RuntimeError. Actionable code suggestions were provided for several functions to ensure robust handling of missing or null list data in the manifest, state, graph, and trace files.