DX-119909: support runtime reload of mutable MCP settings

[aniket-s-kulkarni]

Summary

• add runtime-mutable metadata and safe reload logic for approved MCP settings
• split settings storage into process-wide base settings plus request-scoped overrides
• preserve LaunchDarkly precedence by switching runtime mutable reads to .get(...)
• broaden the MCP refresh loop to reload settings before reevaluating log_level
• add focused regression coverage for reload, override isolation, and mutable subtree materialization

Test Plan

• uv run pytest tests/config/test_settings.py tests/config/test_launchdarkly_integration.py tests/api/test_transport_retry.py tests/api/dremio/test_sql.py tests/servers/test_mcp.py tests/test_fastmcp_basic.py tests/test_simple_fastmcp_server.py tests/tools/test_tools.py tests/tools/test_output_validation.py -q

Jira

• https://dremio.atlassian.net/browse/DX-119909

Reviewer Verdict

• Override accepted for the deployment-specific assumption that runtime config entries are not removed after startup