test(dataset_tests): make tests self-cleaning and remove global-list assertion

[zvika-tonic]

The Python SDK CI matrix (python-version: [3.9, 3.13]) runs both jobs against the same shared PRAPP backend with the same org/API key. This caused cross-matrix flakes in tests/tests/dataset_tests/, most visibly test_get_all_datasets, which:

1. Created two datasets without ever deleting them, leaking state into subsequent runs of the other matrix entry.
2. Asserted len(get_all_datasets()) >= 2, which is a global-list assertion that depends on backend state outside the test's control and can also race with the SDK's internal N+1 list -> get_dataset_by_name loop (a parallel deletion can surface as a 404 mid-loop).

This change:

• Adds tests/tests/dataset_tests/conftest.py with a function-scoped 'created_datasets' fixture: tests append each created dataset name to the yielded list, and the fixture deletes every registered name on teardown (best-effort per name, so a single failure does not mask others). This mirrors the create -> yield -> delete pattern already used by setup_dataset in tests/conftest.py.
• Drops the global len(ds_all) >= 2 assertion in test_get_all_datasets; it now only checks that the two UUID-named datasets it created appear in the result, which is what the test actually intends to verify.
• Wires the new fixture into every backend-touching test in dataset_tests/ that previously created datasets without teardown: test_dataset_lists, test_edit_dataset (all four tests, including the rename case where both pre- and post-rename names are registered), test_get_dataset (both tests), test_get_dataset_files (both tests), test_pii_occurrences (both tests), test_upload_files_to_dataset (all three tests). test_entity_mappings is unaffected (pure unit tests with a mocked client).

Out of scope (tracked separately): the underlying N+1 in tonic_textual/services/dataset.py::get_all_datasets, which is still race-prone against concurrent deletions even with this fix.