`--add-github-mcp-tool=create_pull_request` leaves the configured endpoint readonly

### Describe the bug

`--add-github-mcp-tool` argument still makes the `github-mcp-server` pointing to read only endpoint `/mcp/readonly`
Therefore tool is not added.

### Affected version

1.0.39

### Steps to reproduce the behavior

1. Run copilot with --add-github-mcp-tool=create_pull_request
1. Observe in `github-mcp-server` tool is not added
2. Observe URL pointing to read only endpoint `/mcp/readonly`

### Expected behavior

`--add-github-mcp-tool=create_pull_request` or any other write tool changes the endpoint to `/mcp`

### Additional context

_No response_

Start by tracing the --add-github-mcp-tool argument handling and the github-mcp-server endpoint selection. Reproduce the create_pull_request case, then verify that write tools use /mcp while read-only configuration continues to use /mcp/readonly.

Scritto dal modello di indicizzazione a partire dal testo della issue.