Kiro CLI 2.8.0 - Authentication failed on every prompt after login

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue
- [x] I have run `q doctor` in the affected terminal session
- [x] I have run `q restart` and replicated the issue again

### Operating system

Linux 7.0.0-22-generic - Ubuntu 26.04 LTS

### Expected behaviour

After a successful login, Kiro CLI should answer prompts normally and return model responses.

### Actual behaviour

Every prompt returns:

Authentication failed. Your session may have expired. Run `kiro-cli login` to re-authenticate.

This happens even after:
- logout/login
- removing ~/.kiro
- updating to Kiro CLI 2.8.0
- successful whoami
- successful doctor

### Steps to reproduce

1. Run kiro-cli login
2. Verify with kiro-cli whoami
3. Run kiro-cli chat --legacy-ui
4. Ask any question (e.g. "Who discovered Brazil?")
5. CLI shows "Authentication failed. Your session may have expired."

### Environment

```yaml
OS: Ubuntu 26.04 LTS
Kiro CLI: 2.8.0
Plan: KIRO FREE
Authentication: Builder ID

Commands tested:
- kiro-cli login
- kiro-cli logout
- kiro-cli whoami
- kiro-cli doctor
- kiro-cli chat --legacy-ui

Additional details:
- Login succeeds
- whoami succeeds
- doctor reports "Everything looks good"
- MCP server loads successfully
- Model is shown as "claude-sonnet-4"
- Any prompt results in:
"Authentication failed. Your session may have expired."
```

Start by reproducing the failure with kiro-cli login, whoami, doctor, and chat --legacy-ui on Ubuntu 26.04 with Kiro CLI 2.8.0 and Builder ID authentication. Trace the authentication flow from these CLI entry points; done means a successful login allows prompts to return model responses instead of the authentication error.

Written by the indexing model from the issue text.