Add sortformer to model coverage skip list

The model_coverage_test enforces that every registered model_type has either a
shared test config (tests/_test_configs.py + _registry.py test_model_id) or an
entry in _COVERAGE_SKIP. Sortformer is a NeMo .nemo diarization model with a
custom SortformerConfig and no HuggingFace config.json, so it uses the same
mechanism as the sibling fastconformer_rnnt model: skip the generic coverage
and rely on the dedicated tests/sortformer_integration_test.py (L4/L5 golden)
plus TestBuildGraphSortformer (L1).

Fixes 4 CI failures in tests/model_coverage_test.py for model_type 'sortformer'.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Mason Corey <masoncorey@microsoft.com>

Author: themason2011

tests/model_coverage_test.py

@@ -201,6 +201,7 @@ def _all_registered_with_test_id() -> dict[str, str]:
     "whisper": "Speech-to-text — requires audio inputs",
     "mms": "CTC ASR model — tested via TestBuildMMSGraph",
     "fastconformer_rnnt": "NeMo .nemo RNN-T ASR — tested via tests/nemo_rnnt_integration_test.py",
+    "sortformer": "NeMo .nemo speaker diarization — tested via tests/sortformer_integration_test.py",
     # --- Models requiring trust_remote_code ---
     "chatglm": "Requires trust_remote_code (custom HF modeling code)",
     "dots1": "Requires trust_remote_code (custom HF modeling code)",