Add regression test for the GET /api/v1/runs endpoint

[LudovicoRighi]

#539 moved /runs serialization off the event loop into a background thread (asyncio.to_thread) to stop it blocking the loop on large, unfiltered queries. This fixes the blocking, but introduces a data race.

Why #539 is broken

Before #539, runs_to_jso ran synchronously on the event loop. It contains no await, so it was atomic with respect to every other coroutine — nothing could mutate a run mid-serialization. Slow, but safe.

Under to_thread, serialization runs concurrently with the event loop, which keeps driving the run state machine (_process_updates → Apsis._transition) over the same Run objects being serialized. run_to_summary_jso iterates run.times.items() while Run._transition mutates that dict in place (self.times[state.name] = ...). The result:

RuntimeError: dictionary changed size during iteration

The likelihood scales with exactly the case #539 targets — many runs → long serialization window → more transitions land inside it — so it's not a corner case. (run.meta is aliased into the JSO and also mutated in place by _transition, so it's exposed the same way.)

The test

test/unit/test_runs_endpoint_race.py drives the real api.runs handler in process (mock DB — no server, network, or real Apsis) with a background task transitioning runs, and asserts the handler never crashes.

• Fails on current main with the RuntimeError above.
• Passes if 80a547a is reverted.

The race is probabilistic per call, so the test serves a large run set and repeats the call several times; at these sizes a single buggy call crashes with probability well above 1/2, making a false pass negligible while a correct (atomic) implementation passes every call. It reproduces at Python's default thread-switch interval — no scheduler tuning.

Scope

This PR adds only the failing test to pin down the regression. The fix is deliberately left to follow-up discussion.

cc @gusostow

[LudovicoRighi]

The test fails on latest main (which includes 80a547a)

FAILED test/unit/test_runs_endpoint_race.py::test_runs_endpoint_survives_concurrent_transitions - AssertionError: /runs crashed under concurrent transitions: RuntimeError('dictionary changed size during iteration')
assert RuntimeError('dictionary changed size during iteration') is None

[LudovicoRighi]

And passes after reverting 80a547a