Freeze Python tool timeout in RL requests

[dajiaohuang]

Summary

• Freeze tool_seconds in each RL runtime_contract.
• Validate and use the frozen timeout in worker.plan_round, so local configuration cannot change the published request's tool-call limit.
• Add regression coverage for a conflicting local timeout and reject a modified frozen request.

Fixes #1

Validation

• Focused test on Python 3.12: PYTHONPATH=src uv run --no-project --python 3.12 --with pytest --with jsonschema pytest -q tests/test_explicit_dataset.py -k rl_handoff — 1 passed, 3 deselected.
• The project-resolving invocation is blocked by the checkout's uninitialized editable skyrl submodule; it was not fetched. The full suite and training extra were not run.