What does this PR do?

This PR adds GitHub Enterprise Server (GHE) host support for the copilot provider flow.
Specifically, it makes Copilot auth/runtime host-aware by resolving a GitHub host from:

1. COPILOT_GH_HOST env var
2. config.yaml -> copilot.github_host
3. fallback: github.com
   Then it applies that host in two places:

• Device code login flow (OAuth device endpoints)
• gh auth token lookup (--hostname <host> for GHE)
  In addition, when COPILOT_API_BASE_URL is not explicitly set, Copilot runtime base URL is auto-derived for GHE as:
  https://<host>/api/v3/copilot_internal
  This keeps existing default behavior unchanged for github.com, while enabling out-of-the-box GHE support.

Related Issue

Fixes #11442

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• Updated hermes_cli/copilot_auth.py:
  • added resolve_copilot_github_host() with env+config fallback chain
  • normalized host parsing for URL/plain host forms
  • updated gh auth token flow to pass --hostname for non-default hosts
• Updated hermes_cli/main.py:
  • Copilot device login now uses resolved GitHub host
• Updated hermes_cli/auth.py:
  • added _resolve_copilot_base_url() for GHE base URL derivation
  • integrated this resolver into resolve_api_key_provider_credentials("copilot")
• Added regression tests:
  • tests/hermes_cli/test_copilot_auth.py
  • tests/hermes_cli/test_api_key_providers.py

How to Test

1. Set a GHE host via env or config:
   • env: export COPILOT_GH_HOST=ghe.example.com
   • or config:

     copilot:
       github_host: ghe.example.com

2. Run Copilot model setup/login flow and verify device login uses the GHE host.
3. Verify resolved copilot base URL becomes:
   • https://ghe.example.com/api/v3/copilot_internal
     when COPILOT_API_BASE_URL is unset.
4. Confirm explicit override still wins:
   • COPILOT_API_BASE_URL=https://proxy.example/copilot

Screenshots / Logs

N/A (auth/runtime routing + tests)