[BUG] Claude Desktop crash on broker login

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

Authenticating via the Entra broker flow for a custom third-party MCP server (websearch) causes Claude Desktop to crash on Windows. The app was working correctly on version 1.49585.0. The last log line before the crash is shown below; nothing is logged afterward, and the app crashes silently at this point.

### What Should Happen?

The Entra broker authentication grant should complete successfully and the custom MCP server should authenticate, as it did in 1.49585.0.

### Error Messages/Logs

```shell
2026-09-11 11:53:55 [info] [custom3p-mcp:entra-broker] starting Entra broker grant { server: 'websearch', tenantId: '56ac8205-a37e-4856-a53f-686943febf7b' }
```

### Steps to Reproduce

Configure a custom third-party MCP server (websearch) using broker-based auth (Entra/Azure AD), tenantId 56ac8205-a37e-4856-a53f-686943febf7b.
Trigger authentication (e.g. open the server / send a request requiring auth).
Observe: app crashes immediately after the "starting Entra broker grant" log line, with no further output and no error dialog.

### Claude Model

_No response_

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

1.49585.0

### Claude Code Version

1.52386.0

### Platform

AWS Bedrock

### Operating System

Windows

### Terminal/Shell

PowerShell

### Additional Information

_No response_

No source file or test is named. Start by reproducing the Entra broker flow on Windows with the reported versions and custom websearch MCP server, then compare behavior with 1.49585.0. Done means the broker grant completes, the MCP server authenticates, and Claude Desktop no longer crashes silently.

Written by the indexing model from the issue text.