feat: Add MLflow Prompt Registry provider (squashed commit)

Add a new remote provider that integrates MLflow's Prompt Registry with
Llama Stack's prompts API, enabling centralized prompt management and
versioning using MLflow as the backend.

Features:
- Full implementation of Llama Stack Prompts protocol
- Support for prompt versioning and default version management
- Automatic variable extraction from Jinja2-style templates
- MLflow tag-based metadata for efficient prompt filtering
- Flexible authentication (config, environment variables, per-request)
- Bidirectional ID mapping (pmpt_<hex> ↔ llama_prompt_<hex>)
- Comprehensive error handling and validation

Implementation:
- Remote provider: src/llama_stack/providers/remote/prompts/mlflow/
- Inline reference provider: src/llama_stack/providers/inline/prompts/reference/
- MLflow 3.4+ required for Prompt Registry API support
- Deterministic ID mapping ensures consistency across conversions

Testing:
- 15 comprehensive unit tests (config validation, ID mapping)
- 18 end-to-end integration tests (full CRUD workflows)
- GitHub Actions workflow for automated CI testing with MLflow server
- Integration test fixtures with automatic server setup

Documentation:
- Complete provider configuration reference
- Setup and usage examples with code samples
- Authentication options and security best practices

Signed-off-by: William Caban <william.caban@gmail.com>
Co-Authored-By: Claude <noreply@anthropic.com>

tests/unit/providers/remote/prompts/mlflow/test_mapping.py

@@ -0,0 +1,95 @@
+# 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.
+
+"""Unit tests for MLflow prompts ID mapping utilities."""
+
+import pytest
+
+from llama_stack.providers.remote.prompts.mlflow.mapping import PromptIDMapper
+
+
+class TestPromptIDMapper:
+    """Tests for PromptIDMapper class."""
+
+    @pytest.fixture
+    def mapper(self):
+        """Create ID mapper instance."""
+        return PromptIDMapper()
+
+    def test_to_mlflow_name_valid(self, mapper):
+        """Test converting valid prompt_id to MLflow name."""
+        prompt_id = "pmpt_" + "a" * 48
+        mlflow_name = mapper.to_mlflow_name(prompt_id)
+
+        assert mlflow_name == "llama_prompt_" + "a" * 48
+        assert mlflow_name.startswith(mapper.MLFLOW_NAME_PREFIX)
+
+    def test_to_mlflow_name_invalid(self, mapper):
+        """Test conversion fails with invalid inputs."""
+        # Invalid prefix
+        with pytest.raises(ValueError, match="Invalid prompt_id format"):
+            mapper.to_mlflow_name("invalid_" + "a" * 48)
+
+        # Wrong length
+        with pytest.raises(ValueError, match="Invalid prompt_id format"):
+            mapper.to_mlflow_name("pmpt_" + "a" * 47)
+
+        # Invalid hex characters
+        with pytest.raises(ValueError, match="Invalid prompt_id format"):
+            mapper.to_mlflow_name("pmpt_" + "g" * 48)
+
+    def test_to_llama_id_valid(self, mapper):
+        """Test converting valid MLflow name to prompt_id."""
+        mlflow_name = "llama_prompt_" + "b" * 48
+        prompt_id = mapper.to_llama_id(mlflow_name)
+
+        assert prompt_id == "pmpt_" + "b" * 48
+        assert prompt_id.startswith("pmpt_")
+
+    def test_to_llama_id_invalid(self, mapper):
+        """Test conversion fails with invalid inputs."""
+        # Invalid prefix
+        with pytest.raises(ValueError, match="does not start with expected prefix"):
+            mapper.to_llama_id("wrong_prefix_" + "a" * 48)
+
+        # Wrong length
+        with pytest.raises(ValueError, match="Invalid hex part length"):
+            mapper.to_llama_id("llama_prompt_" + "a" * 47)
+
+        # Invalid hex characters
+        with pytest.raises(ValueError, match="Invalid character"):
+            mapper.to_llama_id("llama_prompt_" + "G" * 48)
+
+    def test_bidirectional_conversion(self, mapper):
+        """Test bidirectional conversion preserves IDs."""
+        original_id = "pmpt_0123456789abcdef" + "a" * 32
+
+        # Convert to MLflow name and back
+        mlflow_name = mapper.to_mlflow_name(original_id)
+        recovered_id = mapper.to_llama_id(mlflow_name)
+
+        assert recovered_id == original_id
+
+    def test_get_metadata_tags_with_variables(self, mapper):
+        """Test metadata tags generation with variables."""
+        prompt_id = "pmpt_" + "c" * 48
+        variables = ["var1", "var2", "var3"]
+
+        tags = mapper.get_metadata_tags(prompt_id, variables)
+
+        assert tags["llama_stack_id"] == prompt_id
+        assert tags["llama_stack_managed"] == "true"
+        assert tags["variables"] == "var1,var2,var3"
+
+    def test_get_metadata_tags_without_variables(self, mapper):
+        """Test metadata tags generation without variables."""
+        prompt_id = "pmpt_" + "d" * 48
+
+        tags = mapper.get_metadata_tags(prompt_id)
+
+        assert tags["llama_stack_id"] == prompt_id
+        assert tags["llama_stack_managed"] == "true"
+        assert "variables" not in tags