feat(flaky-detection): Add pytest-xdist support to flaky detection system

[remyduthu]

Add full flaky detection support under pytest-xdist using controller-
orchestrated pre-computed deadlines and xdist's built-in IPC.

Architecture:

• Controller fetches flaky detection context from API once, distributes
  it to workers via workerinput
• Each worker constructs a FlakyDetector from the serialized context,
  computes the same global budget, runs tests with reruns
• Workers send metrics back via workeroutput
• Controller aggregates metrics and generates the terminal report

Changes:

• Add FlakyDetector.from_context() classmethod for worker-side construction
• Add to_serializable_metrics() for xdist IPC (plain builtins only)
• Add make_report_from_aggregated() for controller-side reporting
• Add static deadline mode (_is_xdist flag) preserving dynamic for non-xdist
• Add MergifyCIInsights.load_flaky_detector_from_context()
• Add controller hooks (pytest_configure_node, pytest_testnodedown)
• Add worker hooks (workerinput loading, workeroutput export)
• Disable flaky detection under 'each' scheduling mode
• Add pytest-xdist dev dependency and integration tests

Fixes: MRGFY-6296

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

Depends-On: #382

[remyduthu]

This pull request is part of a stack:

1. docs: Add design spec for pytest-xdist flaky detection support (#380)
2. docs: Address spec review feedback for xdist flaky detection (#381)
3. docs: Add implementation plan for xdist flaky detection support (#382)
4. feat(flaky-detection): Add pytest-xdist support to flaky detection system (#383) 👈

[mergify]

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