Stamp per-worker-process execution metadata on test results

[bitwise-aiden]

What

Stamps three per-worker-process execution metadata fields on every minitest result, carried nil-safely through TestData#to_h into log/test_data.json:

• parallel_worker_pid — Process.pid at result creation.
• parallel_worker_test_index — 0-based per-process monotonic counter, incremented per execution (requeued runs get their own index). Fork-safe: restarts when the pid changes, so each process incarnation gets a clean 0,1,2,… sequence.
• parallel_worker_id — identifier injected by the embedding environment (e.g. a Rails parallel-testing worker number) via Minitest::Queue.parallel_worker_id= or CI_QUEUE_PARALLEL_WORKER_ID; nil when not applicable.

This makes per-worker-process execution order reconstructable downstream:

PARTITION BY job_id, parallel_worker_id, parallel_worker_pid
ORDER BY parallel_worker_test_index

i.e. a SQL query can reproduce any worker's test_order-w{N}-{pid}.log from warehouse rows — the prerequisite for warehouse-native test-pollution / requeue-overlap analysis.

How

• New Minitest::ParallelWorkerMetadata accessors module prepended to Minitest::Result (fields ride the result object through Marshal/DRb).
• Stamping happens at the top of Minitest::Queue.handle_test_result — in the process that ran the test for all in-process flows.
• First-writer-wins: embedders that run tests in forked workers and transport results to a central reporting process (e.g. Rails parallelization over DRb, where handle_test_result runs server-side) must call Minitest::Queue.stamp_parallel_worker_metadata(result) in the worker before sending. Pre-stamped results pass through reporting untouched — otherwise the reporting-side stamp would carry the server's pid and an arrival-order index interleaved across workers.

Impact on existing consumers

• Fields are additive and nil-safe; TestDataReporter is structurally unchanged.
• rspec-queue, junit.xml, test_order.log, and Redis build-status/error reports are untouched.
• Downstream Monorail wrappers whitelist payload keys, so the new keys are inert until schema/field lists are updated deliberately.

Testing

• New unit suite for stamping (setter/env precedence, fork reset, first-writer-wins, nil-safety).
• TestData unit tests for stamped/unstamped/accessor-less results.
• Integration assertions in test_test_data_reporter: worker id from env, single pid, indexes exactly 0..N-1, requeued execution ordered before its final run.

Also bumps the version to 0.98.0 for release.