[yapdianang]

Summary

• Cache the validated external sampling target for each sampling session.
• Serialize the first lookup so a concurrent request burst performs one validation.
• Keep internal sampling and requests without a sampling session unchanged.

Depends on #2043.

Before / After

Case	Database validation reads
Before: 512 concurrent requests for one sampling session	512
After: 512 concurrent requests for one sampling session	1

The cache stores only a target that passed sampling-session, model, and checkpoint validation. A failed validation is not cached.

Testing

uv run --extra dev --extra jax --extra tinker pytest \
  tests/tinker/test_sampling_target_cache.py \
  tests/tinker/test_api.py
uv run --extra dev pre-commit run --all-files

The API tests pass: 20 passed. All pre-commit hooks pass.

[yapdianang]

Closing because #2061 includes this sampling-target validation change together with the training-request storage fix.