backstage release v0.135.2

AI chat: fix chat requests hanging forever when an MCP server is slow or its responses are dropped by the transport. MCP servers now load in parallel with a per-server timeout (15s default, configurable via aiChat.mcp[].timeoutMs); hanging servers degrade gracefully instead of blocking the chat. Also patch @ai-sdk/mcp to treat SSE events without an explicit event: field as message events per the SSE specification, fixing silently dropped responses from MCP servers behind agentgateway. See ./docs/releases/v0.135.2-changelog.md for more information.