fix(auth): add User-Agent header to Qwen OAuth refresh (closes #7746)

[ten-ltw]

What does this PR do?

Fixes #7746 — Qwen OAuth token refresh silently fails with invalid JSON because the refresh request is missing a User-Agent header. The Qwen OAuth server returns a non-JSON response (HTML / redirect) when the header is absent.

Root Cause

Reported by @gxmst in issue #7746:

• qwen-code-api sends User-Agent: QwenCode/0.14.0 (linux; x64) during token refresh and works fine
• Hermes _refresh_qwen_cli_tokens() sends no User-Agent, causing the OAuth server to return a non-JSON response
• Hermes then calls response.json() on that response and raises a confusing invalid JSON error

Changes

hermes_cli/auth.py:

• Added _QWEN_USER_AGENT = "HermesAgent/1.0" constant
• Added User-Agent: HermesAgent/1.0 to the httpx.post headers in _refresh_qwen_cli_tokens()
• Improved the qwen_refresh_invalid_json error message to include:
  • HTTP status code
  • Content-Type header
  • Response body preview (first 200 chars)

tests/hermes_cli/test_auth_qwen_provider.py:

• test_refresh_qwen_cli_tokens_sends_user_agent_header: verifies the User-Agent header is sent with the correct value
• test_refresh_qwen_cli_tokens_invalid_json_error_includes_diagnostics: verifies diagnostics are surfaced in the error message

How to Test

# 31/31 tests pass (29 existing + 2 new)
uv venv .venv --python 3.11 && source .venv/bin/activate
uv pip install -e ".[all,dev]"
python -m pytest tests/hermes_cli/test_auth_qwen_provider.py -v

# Manual test (requires real Qwen credentials):
hermes qwen auth qwen-oauth
hermes chat  # trigger token refresh

Related Issue

Fixes #7746