fix(dab): mongo crash recovery + vendor common_scaffold for per-query validators

[kentwelcome]

Two independent DAB trial-stability bugfixes, each with a regression test.

1. Mongo crash recovery (compose.py)

mongo:8 intermittently SIGSEGVs (exit 139) on startup, and its WiredTiger cache auto-sizes to ~half of host RAM (≈7GB on a 15GB box), starving the agent. We now:

• Cap the cache: command: ["--wiredTigerCacheSizeGB", "1"]
• restart: on-failure

Because the data dir is already populated, a restarted mongod comes straight back up with data and the healthcheck recovers. Without this, a single crash bricks the whole trial — main's healthcheck fails for its entire retry window and the trial is cancelled.

2. Vendor common_scaffold on the per-query path (prepare.py)

9 of 54 DAB validators do from common_scaffold.validate.levenshtein import levenshtein. verify.py exec_module's validate.py inside the dab-agent container, which has no common_scaffold installed, so the import raised, no reward.json was written, and harbor reported RewardFileNotFoundError (the verifier appeared never to run).

The batch path already vendored common_scaffold next to verify.py; the per-query path now does too (/tests is sys.path[0]), so the import resolves.

Tests

• test_mongo_has_restart_and_cache_cap
• test_per_query_materializes_common_scaffold_for_upstream_validators

All 14 tests in the two affected files pass.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Improves DAB trial stability by hardening MongoDB service behavior in generated docker-compose output and ensuring common_scaffold is vendored for per-query validator execution inside the dab-agent container.

Changes:

• Add restart: on-failure and cap WiredTiger cache size for dab-mongo in generated compose output.
• Vendor common_scaffold into per-query task /tests so upstream validators importing it can run successfully.
• Add regression tests covering both the compose Mongo settings and the per-query common_scaffold materialization.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
packages/razorback-plugin-dab/tests/unit/test_prepare_per_query.py	Adds regression test ensuring per-query tasks vendor common_scaffold and validators can import it.
packages/razorback-plugin-dab/tests/unit/test_compose_mongo.py	Adds regression test asserting Mongo restart policy and WiredTiger cache cap are emitted.
packages/razorback-plugin-dab/src/razorback_plugin_dab/generate/prepare.py	Ensures per-query task materialization also installs common_scaffold into /tests.
packages/razorback-plugin-dab/src/razorback_plugin_dab/generate/compose.py	Updates generated dab-mongo service to restart on failure and cap WiredTiger cache size.

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