fix(sdk-python): detach Run finalizer on finish/exit (fixes #97) - #102

Problem

The Python SDK keeps a process-wide registry (_ACTIVE_RUNS) so two Runs with the same experiment_id can't be live at once. Each Run also arms weakref.finalize(self, _release_active_run, self._experiment_id) as a safety net for a leaked run.

finish() / __exit__ release the slot but never detach() the finalizer. Since the registry is keyed only by experiment_id, GC of an already-finished Run fires its stale finalizer and frees the slot of a different, still-active Run that reused the id:

r1 = Run(experiment_id="x"); r1.finish()   # slot freed
r2 = Run(experiment_id="x")                 # allowed; r2 live, never finished
del r1; gc.collect()                        # r1's stale finalizer clears "x"
r3 = Run(experiment_id="x")                 # SUCCEEDS — guard defeated

r2 and r3 then both run and collide writing EVO_RESULT_PATH — the exact late-failure the registry was added to prevent.

Fix

detach() the finalizer immediately after the slot is released, in both finish()'s finally and __exit__'s exception path. The leaked-run safety net (finalizer fires for a never-finished Run) is unchanged.

Tests

tests/unit/test_sdk_run_finalizer.py (TDD, failing first):

• GC of a finished Run does NOT free a live Run's slot (via finish() and via with)
• a leaked (never-finished) Run still releases its slot on GC

Existing sdk/python/test passes.

Fixes #97.