fix(consolidation): default missing dedup action to keep

[koriyoshi2041]

Summary

• default missing consolidation dedup decisions to keep
• cover the underfilled structured-response path so a near twin is not merged unless the model explicitly returns action="merge"

Fixes #2453.

Tests

• cd hindsight-api-slim && uv run pytest tests/test_consolidation_dedup.py -q
• cd hindsight-api-slim && uv run ruff format --check hindsight_api/engine/consolidation/consolidator.py tests/test_consolidation_dedup.py
• cd hindsight-api-slim && uv run ruff check hindsight_api/engine/consolidation/consolidator.py tests/test_consolidation_dedup.py
• git diff --check

Note: local pre-commit did not complete because control-plane ESLint could not resolve @eslint/js; the focused checks above passed.

[nicoloboschi]

LGTM. Correct conservative fix: defaulting action to keep converts an underfilled structured response from a terminal ValidationError (which aborts the whole consolidation op) into the existing no-merge path. Minimal and right.