fix(puppeteer): return isError instead of internal error on navigation failure

puppeteer_navigate throws an unhandled Protocol error when given an
invalid URL (e.g., empty string, malformed URL). The CDP error
propagates as a JSON-RPC -32603 internal error, which prevents
agents from detecting and recovering from the failure.

Wrap page.goto() in a try/catch and return isError: true with the
error message, matching the pattern used by puppeteer_screenshot
and other tools in this server.

Reproduction:
  mcp-assert audit --server "npx @modelcontextprotocol/server-puppeteer"
  # puppeteer_navigate: CRASH (internal error)

Found with mcp-assert (https://github.com/blackwell-systems/mcp-assert)

Author: blackwell-systems

src/puppeteer/index.ts

@@ -217,7 +217,17 @@ async function handleToolCall(name: string, args: any): Promise<CallToolResult>
 
   switch (name) {
     case "puppeteer_navigate":
-      await page.goto(args.url);
+      try {
+        await page.goto(args.url);
+      } catch (error) {
+        return {
+          content: [{
+            type: "text",
+            text: `Navigation failed: ${error instanceof Error ? error.message : String(error)}`,
+          }],
+          isError: true,
+        };
+      }
       return {
         content: [{
           type: "text",