filesystem: add structured startup validation errors

[Shiyao-Huang]

Summary

Fixes #3512 by adding a machine-readable fatal startup validation error for the filesystem server.

When startup fails because no allowed directory is accessible, the server now emits a stable JSON payload on stderr with:

• type: "startup_validation_error"
• code: "no_accessible_directories"
• message
• rejectedInputs[] containing:
  • input
  • checkedPaths
  • reason (inaccessible or not_directory)

The existing fail-closed behavior remains unchanged, and per-path warning output is preserved.

Files Changed

• src/filesystem/index.ts
• src/filesystem/__tests__/startup-validation.test.ts

Validation

npm run build --workspace @modelcontextprotocol/server-filesystem    # pass
npm run test --workspace @modelcontextprotocol/server-filesystem     # 147 tests passed, 7 files

Notes

• Success paths continue to emit no structured startup error
• Mixed valid + invalid startup inputs remain non-fatal
• Added explicit coverage for the not_directory fatal case

[LuuOW]

Technical audit: Verified MCP server implementation for consistency with current SDK patterns.