fix: silently skip duplicate TestFiles.add() instead of raising #268

Job	Run time
check-linked-issue	2s
determine-changes / detect	10s
coverage	6m 32s
type-check	22s
unit-tests (ubuntu-latest, 3.11)	6m 47s
unit-tests (ubuntu-latest, 3.10)	6m 47s
unit-tests (windows-latest, 3.13)	12m 52s
unit-tests (ubuntu-latest, 3.14)	5m 26s
unit-tests (ubuntu-latest, 3.12)	6m 51s
prek / prek	29s
unit-tests (ubuntu-latest, 3.13)	7m 17s
e2e-python (bubble-sort-pytest-nogit, end_to_end_test_bubblesort_pytest.py, 70, true)	13m 27s
e2e-python (futurehouse-structure, end_to_end_test_futurehouse.py, 5)	3m 8s
unit-tests (ubuntu-latest, 3.9)	8m 49s
e2e-python (tracer-replay, end_to_end_test_tracer_replay.py, 10)	6m 15s
e2e-python (bubble-sort-unittest, end_to_end_test_bubblesort_unittest.py, 40)	12m 30s
e2e-python (coverage-e2e, end_to_end_test_coverage.py, black)	6m 17s
e2e-python (async-optimization, end_to_end_test_async.py, 10)	2m 58s
e2e-python (topological-sort, end_to_end_test_topological_sort_worktree.py, 5)	2m 59s
e2e-python (init-optimization, end_to_end_test_init_optimization.py, 10)	3m 38s
e2e-python (benchmark-bubble-sort, end_to_end_test_benchmark_sort.py, 5)	10m 57s
e2e-js	0s
e2e-java	0s
required checks passed	4s
	2h 4m 37s

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