Preserve recovered project MCP settings when creating the settings file #2175

The JSON persistence caller audit identified a default-write gap in the openProjectMcpSettings handler.

Source: src/core/webview/webviewMessageHandler.ts.

The handler checks whether .roo/mcp.json exists. If it appears absent, it calls safeWriteJson with an empty mcpServers object and no merge. The writer can restore an orphaned committed backup, then replace that restored content with defaults. A peer can also create settings between the existence check and the write.

Proposed scope:

• Decide whether defaults are needed while holding the file lock.
• Preserve valid settings restored from backup or written by a peer.
• Keep the normal empty-file creation behavior.

Acceptance:

• A genuinely missing file gets default settings.
• Opening settings preserves a valid orphaned backup.
• A concurrent creator's settings survive.
• Recovery failures retain backups and remain observable.

Evidence: source inspection. Add regression coverage through the public message handler.