test: capture xdist traces for BigQuery prompt failures

[joeharris76]

Description

Add failure-only worker traces to the fast xdist test run so intermittent BigQuery credential-defaults failures can be diagnosed. The trace keeps the most recent 256 test events per worker, records nodeids and identities for registered modules/globals, and uploads the trace artifact when a test report fails. The leak detector also restores changes to registered Confirm bindings and loaded credential-module identities. Credential behavior is unchanged; the root cause is not yet proven.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation
• Refactor / chore

Testing

• make pr-preflight
• uv run -- python -m pytest tests/unit/test_leak_detector.py tests/unit/platforms/credentials/test_bigquery_defaults.py -n 2 -q
• A controlled xdist failure smoke verified per-worker trace output; the temporary test and artifact were removed.

Public Contract Check

• This PR does not change public, beta-public, deprecated, generated, experimental, or repo-only contract surfaces.

Documentation

• I updated the relevant user-facing docs.
• I added regression notes for behavior changes.
• API contract wording remains accurate because no API changed.

Code Quality

• I ran lint, formatting, and changed-file tests.
• I reviewed impacted modules for backwards compatibility and migration impact.
• I added/updated tests for the process-state guard.
• Public contracts and release checklists remain unchanged.

Artifact Hygiene

• No raw screenshots, logs, benchmark outputs, or temporary artifacts were committed.

[benchbox-oracle]

oracle-verdict: success for df9bdbf03c81f9824e9d56c746c63fe3c48f4c18

This result supersedes those for earlier heads; their open review threads still apply.

Tier: low-medium (data-only or small single-area change).

Decision: SHIP.

Reviewer: muse (muse muse-spark-1.3-contributor, effort medium).

Reviewed Confirm leak tracking, sys.modules replacement detection/restoration, xdist failure tracing, and CI artifact upload. New tracking strictly strengthens the hermetic gate, first-import exemption is intentional and tested, trace records only ids/nodeids with no secret values, restoration handles sys.modules plus parent attribute, and CI changes are inert without xdist workers. No merge-blocking correctness, verifiability, gate-weakening, or security defect found.

Reviewers not available

• luna: absent (error)

[benchbox-oracle]

oracle-verdict: success for 4a9f3a3ca8c34dbaa64489d56eabba1e1ca96bb2

This result supersedes those for earlier heads; their open review threads still apply.

Tier: low-medium (data-only or small single-area change).

Decision: SHIP, carried from head df9bdbf03c81f9824e9d56c746c63fe3c48f4c18.

The patch is unchanged since head df9bdbf03c81f9824e9d56c746c63fe3c48f4c18 was reviewed, so its decision is carried to this head without running a reviewer. Its open review threads still apply.