Resilient tool merging across multiple MCP servers

This patch release prevents a single failing MCP server from wiping out tools from healthy ones in multi-MCP chat configurations.

One failing MCP server no longer drops tools from healthy ones #2816 - When a chat is configured with multiple MCP servers, a rejection on any single tools/list call (e.g. a 401 on one toolset) used to wipe out the merged tool map and trigger React Query retries against every server. Healthy servers are now merged in normally; only when every server fails does the query reject and retry. (Author: @danielkov)