fix(consolidation): default invalid dedup actions to keep

[r266-tech]

Summary

Fixes #2545.

Consolidation dedup already defaulted a missing structured action to keep, but near-miss or malformed action values from a lax structured-output model could still fail Pydantic validation before the conservative no-merge path ran.

This adds a mode="before" validator on _DedupDecision.action that:

• preserves exact "merge" / "keep" decisions
• logs invalid action values
• defaults every other scalar or non-scalar shape to "keep"

That keeps malformed dedup verdicts from accidentally merging observations or requeueing the consolidation job.

Tests

• ruff check hindsight_api/engine/consolidation/consolidator.py tests/test_consolidation_dedup.py
• ruff format --check hindsight_api/engine/consolidation/consolidator.py tests/test_consolidation_dedup.py
• python3 -m py_compile hindsight_api/engine/consolidation/consolidator.py tests/test_consolidation_dedup.py
• local validator smoke check for exact valid actions, unknown string actions, near-miss Merge, list/dict values, and missing action

I could not run pytest tests/test_consolidation_dedup.py in this macOS arm64 environment: uv run resolves onnxruntime==1.27.0, which has no wheel for the current Python/platform combo, and direct PYTHONPATH=. pytest ... -o addopts='' cannot collect without the project runtime deps such as asyncpg installed.

Codex review noted a low-confidence import concern about field_validator; the module already imports it from Pydantic and py_compile passed.

[benfrank241]

lgtm. defaulting anything that isn't exactly merge/keep to keep is the right conservative call. Merging. Thanks!