[BUG] claudecode.nvim cannot open

## Bug Description

After running "Toggle Claude", just a blank pane appear.

## To Reproduce

Steps to reproduce the behavior:
1. running "Toggle Claude"
2. hit `enter`,
3. See error

## Expected Behavior

a pane display claude code.

## Environment

- Neovim version: v0.11.4
- Claude Code CLI version: 1.0.113
- OS: Windows 11 24H2
- Plugin version: 0.2.0

## Error Messages

If applicable, add error messages or logs.

```
[ClaudeCode] [terminal] [ERROR] Claude exited with code 4000.
Check for any errors.
```

## Additional Context

I am using `https://open.bigmodel.cn/api/anthropic` as `$env:ANTHROPIC_BASE_URL`. And it work in native claude code.

The issue names no source file or test; start by reproducing the `Toggle Claude` flow in Neovim on Windows with the stated Claude Code and ANTHROPIC_BASE_URL environment. Trace the terminal error and verify that the pane displays Claude Code instead of exiting with code 4000.

Written by the indexing model from the issue text.