Tracking #1380: MCP settings stub write merged via safeWriteJson (race #1371)

[easonLiangWorldedtech]

Tracking issue for upstream PR #1380 (branch fix/mcp-settings-stub-race-1371).

Part of the file-write safety series — plan: easonLiangWorldedtech/Zoo-Code#33, epic: #1375. Related upstream issue: #1371.

Scope

• McpHub.getMcpSettingsFilePath() now writes the MCP settings stub via safeWriteJson(..., { prettyPrint: true, merge }) so an existing file is read-merged under the advisory lock instead of being blindly overwritten (no lost mcpServers keys on the first stub write).
• Coverage tests for the merge fallback (codecov patch 100% — all changed lines + BRDA branches).

Acceptance

• Stub write preserves existing keys via the merge callback
• CI green (ubuntu unit-test gate)
• Bot comments resolved

Status

• PR: OPEN, MERGEABLE — CI green (platform-unit-test ubuntu SUCCESS, codecov/patch SUCCESS, CodeRabbit SUCCESS)