@W-22065947: Allow for disabling the MCP server globally

[anyoung-tableau]

These changes add a BREAK_GLASS_DISABLE_GLOBALLY env var that allows for disabling then MCP server globally. When true all tool call results will immediately return an error.

The motivation here is in the event there is some significant livesite issue that warrants bringing the whole server down, we can do so while still handling requests and returning a somewhat helpful error message to clients.

[Copilot]

Pull request overview

Adds a “break glass” environment-variable switch to globally disable MCP tool execution while keeping the server process up, intended for mitigating live-site incidents with a clearer client-facing failure mode.

Changes:

• Introduces BREAK_GLASS_DISABLE_GLOBALLY config/env plumbing and a new ServiceUnavailableError (503).
• Short-circuits tool callbacks in Server.registerTools() when the flag is enabled.
• Updates tests and documentation to cover/describe the new global-disable behavior.

Reviewed changes

Copilot reviewed 11 out of 13 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
types/process-env.d.ts	Adds the new env var to the typed process env surface.
src/config.ts	Parses BREAK_GLASS_DISABLE_GLOBALLY into config.breakGlassDisableGlobally.
src/config.test.ts	Adds default/true parsing tests for the new config flag.
src/errors/mcpToolError.ts	Adds ServiceUnavailableError (type service-unavailable, 503).
src/server.ts	Rejects tool calls when breakGlassDisableGlobally is enabled.
src/server.test.ts	Switches tests to vi.stubEnv + adds break-glass rejection test.
src/tools/listDatasources/listDatasources.ts	Exposes params schema via exportedForTesting for use in server test.
src/index.ts	Emits a stderr warning when break-glass is enabled at startup.
docs/docs/enterprise/tableau-server.md	Documents how to temporarily disable service via the new env var.
docs/docs/configuration/mcp-config/env-vars.md	Adds documentation section for BREAK_GLASS_DISABLE_GLOBALLY.
package.json / package-lock.json	Bumps version to 1.18.6.

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