Add srt-slurm MCP spec server and preflight validation

[ishandhanani]

Summary

• add a minimal FastMCP server for srt-slurm schema/docs/validation access
• add model/container preflight validation and wire it into submit paths
• cover the new MCP and validation behavior with tests

Testing

• uv run --with pytest pytest tests/test_validation.py tests/test_mcp_spec.py tests/test_apply_json.py tests/test_apply_mock.py tests/test_override.py -q