tests: keep GC pauses out of the node-deadline timing window

[DaveHomeAssist]

Problem

tests/test_node_deadlines.py::test_complete_deadline_interrupts_real_http_reads[503-body] failed on every Python 3.14 CI run of #51 (attempts 1 and 2) and #52 (attempts 1 and 2). It took 0.83–0.99 s against its 0.7 s bound. main, #47 and #53 passed. All runs used CPython 3.14.7, and the test passes alone locally.

The cause is a garbage-collection pause, not the deadline code. A local full suite on 3.14 with a gc.callbacks probe logged 31 collections over 30 ms: up to 1.04 s for gen 2 and 0.5–0.7 s for several gen 1 passes. Those pauses are larger than the 0.45 s of slack between TOTAL (0.25 s) and UPPER_BOUND (0.7 s). Collections fall at deterministic points in the suite, and the tests #51 and #52 add moved one into [503-body].

Change

slow_node disables automatic GC while its server is up and restores the previous state on exit. That covers every timed assertion in this file, including the error_headers case, which has only 0.25 s of slack. Deadline code, TOTAL, UPPER_BOUND and the server stall times are unchanged, so the tests still prove the same bounds.

Checks

• pytest -q tests/test_node_deadlines.py: 13 passed.
• Full suite with the GC probe: see the follow-up comment.
• git diff --check: clean.

After this merges, #51 and #52 need main merged in to rerun CI.

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[DaveHomeAssist]

Local full suite on this branch (Python 3.14.3, with the gc.callbacks probe): 757 passed, 1 skipped. The probe logged 28 collections over 30 ms elsewhere in the suite (up to 0.57 s, gen 2), and 0 inside test_node_deadlines.py. The same probe on #52's tree before the fix: 31 collections over 30 ms, up to 1.04 s.