feat: Add MLflow Prompt Registry provider

This commit adds a new remote provider for the Prompts API that integrates
with MLflow's Prompt Registry (MLflow 3.4+), enabling centralized prompt
management and versioning.

Key features:
- Full CRUD operations (create, read, update, list)
- Immutable version control with default version management
- Automatic variable extraction from {{ variable }} placeholders
- Deterministic ID mapping between Llama Stack and MLflow
- Comprehensive test coverage (41 unit + 14 integration tests)

Provider implementation:
- src/llama_stack/providers/remote/prompts/mlflow/
  - config.py: Configuration schema with validation
  - mapping.py: ID mapping and metadata utilities
  - mlflow.py: Main adapter implementation (520 lines)
- src/llama_stack/providers/registry/prompts.py: Provider registration

Testing:
- tests/unit/providers/remote/prompts/mlflow/
  - test_config.py: 14 configuration tests
  - test_mapping.py: 27 ID mapping tests
- tests/integration/providers/remote/prompts/mlflow/
  - test_end_to_end.py: 14 end-to-end scenarios
  - conftest.py: Test fixtures and server availability checks
  - README.md: Testing documentation
- scripts/test_mlflow_prompts_manual.py: Manual validation script

Documentation:
- docs/docs/providers/prompts/remote_mlflow.mdx: User guide
- docs/docs/providers/prompts/index.mdx: Provider category index

Dependencies:
- Requires mlflow>=3.4.0 (added to provider pip_packages)

Limitations:
- No deletion support (MLflow constraint)
- Sequential version numbers only (1, 2, 3...)

Signed-off-by: William Caban <[email protected]>
Co-Authored-By: Claude <[email protected]>

Author: williamcaban

docs/docs/providers/prompts/index.mdx

@@ -0,0 +1,12 @@
+---
+sidebar_label: Prompts
+title: Prompts
+---
+
+# Prompts
+
+## Overview
+
+This section contains documentation for all available providers for the **prompts** API.
+
+The Prompts API enables centralized management of prompt templates with versioning, variable handling, and team collaboration capabilities.