Harden task serialization failures

[vikranthreddimasu]

Summary

• Pre-serialize outbound task specs before registering dispatcher pending state, so non-msgpack-native arguments fail immediately without leaking pending futures.
• Convert worker result serialization failures into structured remote task errors instead of letting the worker reply coroutine die and strand the coordinator.
• Added regression coverage for invalid task arguments and invalid worker return values.

Security changes

• No auth, credential, TLS, or authorization policy changes.
• Added an audit event for worker result serialization failures, with only task metadata and exception type.

Usability changes

• Callers now get explicit serialization errors instead of timeouts when task payloads or return values cannot cross the wire.

Tests/checks run

• python -m pytest tests/test_compute/test_dispatch.py -v
• python -m pytest tests/test_compute tests/test_fuzz/test_fuzz_task_spec.py -v
• ruff check macfleet/compute/dispatch.py macfleet/compute/worker.py tests/test_compute/test_dispatch.py
• ruff check macfleet/compute tests/test_compute tests/test_fuzz/test_fuzz_task_spec.py
• make lint
• make test
• GitHub Actions test workflow: all 11 jobs passed
• git diff --check master...HEAD
• git diff master...HEAD | rg -n "(BEGIN .*PRIVATE KEY|AWS_SECRET_ACCESS_KEY|GITHUB_TOKEN|password=|token=|secret=|sk-[A-Za-z0-9])" || true

Commits

• 8b7dfe4 Harden task serialization failures

Risks / manual review notes

• Low risk. Change is limited to remote task dispatch/worker serialization error handling and tests.
• make test passed with 1025 tests and 2 existing legacy cloudpickle deprecation warnings.

Merge status

• Merged into master after all checks passed.