test(delegation): isolate and retire fixture-owned Effect runtimes

[songoow]

Goal And Delivered Outcome

• Outcome basis: test-infrastructure repair ported from closed feat(delegation): stop delegated members with acknowledged, settled receipts #5308.
• Goal/source and gap: the delegation service fixture changed TMPDIR/TEMP/TMP for its children but left Python's cached tempfile.tempdir untouched, so a warmed parent could route to a different Effect runtime than its native CLI children. It also left each case's private runtime alive until the five-minute idle shutdown, accumulating servers across the suite, including failed setups.
• Observable before → after: before, the extended isolation test failed on the old fixture. After, it passes. A neighboring runtime survives teardown.

Author Declaration

• Written by: model_agent — Claude Fable 5.1 (Anthropic), operated by @songoow.

Implemented against

• Specification and revision: no written specification; the commit ported from feat(delegation): stop delegated members with acknowledged, settled receipts #5308 is the basis. The contract PR docs(rfc): specify the lease-fenced delegation stop contract #5534 cites the fixture isolation change as a prerequisite.
• Criteria: N/A (test-infrastructure only).

Criterion (spec clause)	Disposition	Symbol / path	Test or command
N/A	N/A	tests/test_local_delegation.py	uv run --extra test python -m pytest -q tests/test_local_delegation.py -k isolates_cached_and_child_runtime_routes

• Self-check before submission: ran the isolation test on main (fails), applied the patch (passes), ran the whole delegation test file (green).

Scope And Continuation

• Completed scope and remaining work: fixture isolation is complete. The stop-feature PR depends on this.
• Slice boundary / successor: docs(rfc): specify the lease-fenced delegation stop contract #5534 (contract) + the stop-feature PR, logically after this.

Validation

• Tested revision: 759349919
• Run state: finished
• Input classes: File and SQLite delegation service fixtures

Check kind	Result	Public-safe evidence / limitation
smoke	passed	tests/test_local_delegation.py::test_delegation_fixture_isolates_cached_and_child_runtime_routes on the fixed fixture (was failing on main).
suite	passed	Whole tests/test_local_delegation.py (~32 cases).

• Coverage and gaps: test-infrastructure only, no product runtime change.

See validation disclosure guidance.

Frontend / Visual Evidence

• UI impact: none

Type of Change

• Tests

LoopX Area

• Control plane (goals, todos, quota, scheduler, registry, runtime)

Technical Direction

• Direction / acceptance reference: test infrastructure repair, ported from closed feat(delegation): stop delegated members with acknowledged, settled receipts #5308.

Shared-authority RFC fixture impact

• N/A: test-infrastructure only; no fixture, provider, projection or runtime routing change in production code.

Boundary Checklist

• Neither the diff nor this PR body/comments/attachments disclose private state, credentials, raw traces or verifier output, internal links, or local machine paths.
• I did not duplicate maintainer-owned benchmark work unless a maintainer split out a public issue for it.
• I kept the change scoped to the linked issue/task.
• I completed the visual evidence section for UI changes, or marked UI impact none.
• Every commit includes a DCO Signed-off-by trailer (git commit -s).

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