refactor: make sku_list resolve provider aliases generically

- Replace Together-specific import logic with generic provider prefix stripping
- Addresses PR #2796 review feedback about provider-specific code in core modules
- Add comprehensive test suite covering normal cases, provider prefixes, and edge cases
- Maintains backward compatibility while keeping sku_list provider-agnostic

tests/unit/models/test_sku_resolve_alias.py

@@ -0,0 +1,68 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+from llama_stack.models.llama.sku_list import resolve_model
+
+
+def test_resolve_by_descriptor():
+    """Test normal resolution by model descriptor."""
+    model = resolve_model("Llama-4-Scout-17B-16E-Instruct")
+    assert model is not None
+    assert model.core_model_id.value == "Llama-4-Scout-17B-16E-Instruct"
+
+
+def test_resolve_by_huggingface_repo():
+    """Test normal resolution by HuggingFace repo path."""
+    model = resolve_model("meta-llama/Llama-4-Scout-17B-16E-Instruct")
+    assert model is not None
+    assert model.core_model_id.value == "Llama-4-Scout-17B-16E-Instruct"
+
+
+def test_together_alias_resolves():
+    """Test that Together-prefixed alias resolves via generic prefix stripping."""
+    alias = "together/meta-llama/Llama-4-Scout-17B-16E-Instruct"
+    model = resolve_model(alias)
+    assert model is not None, f"Model should resolve for alias {alias}"
+    assert model.core_model_id.value == "Llama-4-Scout-17B-16E-Instruct"
+
+
+def test_groq_alias_resolves():
+    """Test that Groq-prefixed alias resolves via generic prefix stripping."""
+    alias = "groq/meta-llama/Llama-4-Scout-17B-16E-Instruct"
+    model = resolve_model(alias)
+    assert model is not None, f"Model should resolve for alias {alias}"
+    assert model.core_model_id.value == "Llama-4-Scout-17B-16E-Instruct"
+
+
+def test_unknown_model_returns_none():
+    """Test that unknown model descriptors return None."""
+    model = resolve_model("nonexistent-model")
+    assert model is None
+
+
+def test_unknown_provider_prefix_returns_none():
+    """Test that unknown provider prefix with unknown model returns None."""
+    model = resolve_model("unknown-provider/nonexistent-model")
+    assert model is None
+
+
+def test_empty_string_returns_none():
+    """Test that empty string returns None."""
+    model = resolve_model("")
+    assert model is None
+
+
+def test_slash_only_returns_none():
+    """Test that just a slash returns None."""
+    model = resolve_model("/")
+    assert model is None
+
+
+def test_multiple_slashes_handled():
+    """Test that paths with multiple slashes are handled correctly."""
+    # This should strip "provider/" and try "path/to/model"
+    model = resolve_model("provider/path/to/model")
+    assert model is None  # Should be None since "path/to/model" doesn't exist