Isolate missing-env reporting state tests

[karthiknadig]

Problem

Windows ARM CI intermittently fails four pet::jsonrpc missing-env-reporting tests. The tests serialize against each other with MISSING_ENVS_TEST_LOCK, but unrelated configure tests concurrently reset the same process-global MISSING_ENVS_REPORTING_STATE. One assertion then fails while holding the mutex, poisoning it and cascading into the remaining failures.

Observed in PR #499 job 92449574269.

Acceptance criteria

• Extract missing-env state-machine helpers that accept an injected AtomicU64.
• Keep production wrappers using the process-global state.
• Update unit tests to use independent local atomics and remove the global test mutex.
• Run the affected tests repeatedly and in parallel with configure tests.