feat: allow user to register model alias explicitly, tests

# What does this PR do?

Context: https://github.com/llamastack/llama-stack/discussions/3483

This PR enables the registering `provider_model_id` as the model identifier without breaking backward compatibility.


## Test Plan
todo
# What does this PR do?


## Test Plan

tests/unit/distribution/routers/test_routing_tables.py

@@ -645,3 +645,88 @@ async def test_models_source_interaction_cleanup_provider_models(cached_disk_dis
 
     # Cleanup
     await table.shutdown()
+
+
+async def test_models_register_with_use_provider_model_id_as_id(cached_disk_dist_registry):
+    """Test register_model with the new use_provider_model_id_as_id parameter."""
+    table = ModelsRoutingTable({"test_provider": InferenceImpl()}, cached_disk_dist_registry, {})
+    await table.initialize()
+
+    # Register model using use_provider_model_id_as_id parameter
+    await table.register_model(
+        provider_model_id="actual-provider-model", provider_id="test_provider", use_provider_model_id_as_id=True
+    )
+
+    # Verify the model was registered with provider_model_id as identifier
+    models = await table.list_models()
+    assert len(models.data) == 1
+    model = models.data[0]
+    assert model.identifier == "actual-provider-model"
+    assert model.provider_resource_id == "actual-provider-model"
+    assert model.provider_id == "test_provider"
+
+    # Test lookup by provider_model_id works
+    retrieved_model = await table.get_model("actual-provider-model")
+    assert retrieved_model.identifier == "actual-provider-model"
+    assert retrieved_model.provider_resource_id == "actual-provider-model"
+
+    # Cleanup
+    await table.shutdown()
+
+
+async def test_models_register_provider_model_id_only(cached_disk_dist_registry):
+    """Test register_model with only provider_model_id (new recommended usage)."""
+    table = ModelsRoutingTable({"test_provider": InferenceImpl()}, cached_disk_dist_registry, {})
+    await table.initialize()
+
+    # Register model using only provider_model_id
+    await table.register_model(provider_model_id="llama-3.1-8b", provider_id="test_provider", model_type=ModelType.llm)
+
+    # Verify the model was registered with namespaced identifier
+    models = await table.list_models()
+    assert len(models.data) == 1
+    model = models.data[0]
+    assert model.identifier == "test_provider/llama-3.1-8b"
+    assert model.provider_resource_id == "llama-3.1-8b"
+    assert model.provider_id == "test_provider"
+
+    # Test lookup works
+    retrieved_model = await table.get_model("test_provider/llama-3.1-8b")
+    assert retrieved_model.identifier == "test_provider/llama-3.1-8b"
+
+    # Cleanup
+    await table.shutdown()
+
+
+async def test_models_register_validation_errors(cached_disk_dist_registry):
+    """Test register_model validation errors."""
+    table = ModelsRoutingTable({"test_provider": InferenceImpl()}, cached_disk_dist_registry, {})
+    await table.initialize()
+
+    # Test error when neither model_id nor provider_model_id is provided
+    with pytest.raises(ValueError, match="provider_model_id must be provided"):
+        await table.register_model(provider_id="test_provider")
+
+    # Cleanup
+    await table.shutdown()
+
+
+async def test_models_register_backward_compatibility_warning(cached_disk_dist_registry):
+    """Test that register_model warns when model_id equals provider_model_id."""
+    from unittest.mock import patch
+
+    table = ModelsRoutingTable({"test_provider": InferenceImpl()}, cached_disk_dist_registry, {})
+    await table.initialize()
+
+    # Test warning is logged when model_id == provider_model_id
+    with patch("llama_stack.core.routing_tables.models.logger") as mock_logger:
+        await table.register_model(model_id="same-model", provider_model_id="same-model", provider_id="test_provider")
+
+        # Verify warning was called
+        mock_logger.warning.assert_called_once()
+        warning_msg = mock_logger.warning.call_args[0][0]
+        assert "model_id` is now optional" in warning_msg
+        assert "provider_model_id='same-model'" in warning_msg
+
+    # Cleanup
+    await table.shutdown()