fix(mcp): handle missing Tavily API key gracefully

Previously, when websearch was configured with Tavily provider and the
TAVILY_API_KEY environment variable was not set, the entire plugin would
fail to load with no visible error to the user.

Changes:
1. createWebsearchConfig now returns undefined when Tavily key is missing
2. Added warning log: '[websearch] Tavily API key not found, skipping websearch MCP'
3. createBuiltinMcps now skips undefined configs instead of adding them
4. Added tests for both missing and present Tavily API key scenarios

Fixes #2996

src/mcp/websearch.ts

@@ -1,4 +1,5 @@
 import type { WebsearchConfig } from "../config/schema"
+import { log } from "../shared/logger"
 
 type RemoteMcpConfig = {
   type: "remote"
@@ -14,7 +15,8 @@ export function createWebsearchConfig(config?: WebsearchConfig): RemoteMcpConfig
   if (provider === "tavily") {
     const tavilyKey = process.env.TAVILY_API_KEY
     if (!tavilyKey) {
-      throw new Error("TAVILY_API_KEY environment variable is required for Tavily provider")
+      log("[websearch] Tavily API key not found, skipping websearch MCP")
+      return undefined
     }
 
     return {