Add mcp support for finetuning comp

[dittops]

PR Description

This PR adds MCP support to the finetuning microservice and ships MCP validation tests. It registers finetuning endpoints as MCP tools, adds a base64 upload tool for MCP clients, and ensures job status updates run without FastAPI BackgroundTasks in MCP mode. It also adds MCP dependencies, updates compose/docs, and includes a new MCP test script.

Key changes

• MCP enablement: ENABLE_MCP toggle added to finetuning microservice; tool registration for create/list/retrieve/cancel
  jobs + list checkpoints.
• MCP upload tool: New upload_training_files_mcp accepts base64 file payloads.
• Background tasks in MCP: schedule_job_status_update fallback uses a daemon thread when BackgroundTasks isn’t
  available.
• Deps/compose/docs: Added mcp dependencies, updated compose env, and documented MCP usage.
• Tests: Added tests/finetuning/validate_mcp.py and tests/finetuning/test_finetuning_mcp.sh (venv-based install to
  avoid PEP 668).