feat(mcp): broker read tools

[andypalmi]

Summary

Adds read-only MCP tools for the team broker (built-in MQTT broker) and 3rd-party broker surface in forge/ee/lib/mcp/tools/broker.js:

• platform_list_broker_clients - GET /api/v1/teams/:teamId/broker/clients
• platform_get_broker_client - GET /api/v1/teams/:teamId/broker/client/:username
• platform_list_brokers - GET /api/v1/teams/:teamId/brokers
• platform_get_broker - GET /api/v1/teams/:teamId/brokers/:brokerId
• platform_list_broker_topics - GET /api/v1/teams/:teamId/brokers/:brokerId/topics
• platform_get_broker_schema - GET /api/v1/teams/:teamId/broker/:brokerId/schema

All six tools carry readOnlyHint: true / destructiveHint: false and none of them return raw broker credentials.

Allow-lists the backing scopes in IMPLICIT_TOKEN_SCOPES['user:expert-mcp'] (forge/routes/auth/permissions.js):

• broker:clients:list
• broker:credentials:list
• broker:topics:list

This scope is under special review (credential-sensitive category), so only these read tools are proposed here. The credential-issuing endpoints (broker client create/link, client password update, 3rd-party broker registration, raw credential fetch) and the non-credential write/delete surface (topic CRUD, lifecycle, delete client) are intentionally out of scope for this PR.

Closes #7711

Test plan

• eslint clean on forge/ee/lib/mcp/tools/broker.js and forge/routes/auth/permissions.js
• Endpoint URLs verified against the route definitions in forge/ee/routes/teamBroker/index.js, forge/ee/routes/teamBroker/3rdPartyBroker.js, and forge/ee/routes/teamBroker/schema.js

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.61%. Comparing base (103f9ae) to head (cb1cf5c).

Additional details and impacted files

@@                        Coverage Diff                        @@
##           feat/mcp-tools-shared-schemas    #7723      +/-   ##
=================================================================
+ Coverage                          75.41%   75.61%   +0.20%     
=================================================================
  Files                                426      427       +1     
  Lines                              22551    22574      +23     
  Branches                            5949     5949              
=================================================================
+ Hits                               17006    17069      +63     
+ Misses                              5545     5505      -40     

Flag	Coverage Δ
backend	75.61% <100.00%> (+0.20%)	⬆️

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