Add Mason MCP service discovery

[junchoi-db]

What changed

• add mason mcp list to discover MCP Services visible in system.ai
• support --schema catalog.schema for MCP Services in another UC schema
• provide copyable mason tools add mcp ... commands and stable JSON output
• remove the legacy top-level mason add-sandbox alias; mason tools add sandbox remains supported

Design doc: n/a

Verification

• ruff format --check .
• ruff check .
• ty check src/databricks_mason/mcp.py tests/unit_tests/mcp_test.py
• pytest -q — 158 passed
• wheel build: databricks_mason-0.1.0.dev0-py3-none-any.whl

Live E2E proof

Tested the built wheel, not the source checkout:

uvx --from /tmp/mason-dist/databricks_mason-0.1.0.dev0-py3-none-any.whl \
  mason -p df1 mcp list
uvx --from /tmp/mason-dist/databricks_mason-0.1.0.dev0-py3-none-any.whl \
  mason -p dogfood mcp list

Both commands exited 0 and listed 11 services with copyable add commands:

system.ai.atlassian
system.ai.dbsql
system.ai.genie_one_mcp
system.ai.github
system.ai.gmail
system.ai.google_calendar
system.ai.google_drive
system.ai.microsoft_365
system.ai.sandbox
system.ai.slack
system.ai.web_search

JSON output was separately asserted on both profiles: schema_version == 1, count 11,
and known system.ai.web_search and system.ai.sandbox entries present.

This pull request and its description were written by Isaac.