feat: swev-id: pytest-dev__pytest-5787 preserve xdist exception chains

[casey-brooks]

Summary

• carry ExceptionChainRepr data through BaseReport JSON helpers
• reconstruct chained exceptions during deserialization while keeping legacy path intact
• add regression coverage for JSON, hook, and xdist round-trips

Testing

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 . .venv/bin/activate && pytest testing/test_reports_chain.py -q
• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 . .venv/bin/activate && pytest testing/test_reports.py -q
• . .venv/bin/activate && flake8 src/_pytest/reports.py testing/test_reports_chain.py

Reproduction

Reproducer from #16:

def test_chained_exception_with_from():
    try:
        try:
            raise ValueError(11)
        except Exception as e1:
            raise ValueError(12) from e1
    except Exception as e2:
        raise ValueError(13) from e2


def test_chained_exception_without_from():
    try:
        try:
            raise ValueError(21)
        except Exception:
            raise ValueError(22)
    except Exception:
        raise ValueError(23)

Before this change, running with pytest -n auto would only show E ValueError: 13 (or E ValueError: 23) on the controller, while a local run displayed the full chain. After the fix, the xdist run includes the cause/context banners ("During handling ..." / "The above exception ...") the same as a local run.

Resolves #16

[casey-brooks]

Local Verification

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 . .venv/bin/activate && pytest testing/test_reports_chain.py -q → 3 passed
• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 . .venv/bin/activate && pytest testing/test_reports.py -q → 14 passed
• . .venv/bin/activate && flake8 src/_pytest/reports.py testing/test_reports_chain.py → no findings

[noa-lucent]

Looks good. Carrying the chain information through _to_json/_from_json restores the missing context for xdist, and the new regression tests cover the JSON, hook, and worker/controller paths.