fix(mcp): preserve concurrent MCP settings during initial creation (fixes #1371)

[easonLiangWorldedtech]

Summary

Fixes Zoo-Code-Org#1371 (part of epic Zoo-Code-Org#1375).

MCP settings initialization has a check-then-create race: getMcpSettingsFilePath() did fileExistsAtPath() followed by an unconditional fs.writeFile of the empty stub. Two VS Code windows (separate extension-host processes) racing at startup both see the file as absent, and the second blind write truncates the first window's config to the 122-byte stub — silent data loss of all configured MCP servers.

Changes

• src/services/mcp/McpHub.ts — the stub creation in getMcpSettingsFilePath() now goes through safeWriteJson with a merge callback:
  • the merge read happens under the existing advisory lock, so any config written by a concurrent process after the existence check is visible to it;
  • if the on-disk content has a proper mcpServers object, it is preserved; otherwise the default stub is written;
  • the fast path is unchanged (file exists → no write at all), so no new writes, no mtime churn, no watcher-triggered MCP reloads.
• src/services/mcp/__tests__/McpHub.spec.ts:
  • regression test reproduces the interleaving: the existence check sees an absent file, but the locked read sees a concurrent window's config; asserts the creation write carries that config, not the stub. (This test fails against the old raw fs.writeFile implementation.)
  • the safeWriteJson spec mock now honors options.merge (it previously ignored options, which masked merge semantics for every test in this file).

Notes

• safeWriteJson was already the established pattern for this file (used by the add/remove-server flows); this change removes the one remaining raw fs.writeFile in McpHub.
• No suppression count changes; eslint --max-warnings=0 clean on both touched files.
• pnpm --dir src exec vitest run services/mcp/__tests__/McpHub.spec.ts → 64/64 passing.

[easonLiangWorldedtech]

opened as cross-repo upstream PR instead