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/integration/providers/remote/prompts/__init__.py

@@ -0,0 +1,7 @@
+# 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.
+
+"""Integration tests for remote prompts providers."""