feat(extensions,presets): authenticate GitHub-hosted catalog and download requests with GITHUB_TOKEN/GH_TOKEN (#2331)

* feat(extensions,presets): authenticate GitHub-hosted catalog and download requests with GITHUB_TOKEN/GH_TOKEN

Squashed from #2087 (original author: @anasseth).

Adds GitHub-token authentication to extension and preset catalog fetching
and ZIP downloads so private GitHub repos work when GITHUB_TOKEN/GH_TOKEN
is set, while preventing credential leakage to non-GitHub hosts.

- Introduces shared _github_http module with build_github_request() and
  open_github_url() helpers
- Routes ExtensionCatalog and PresetCatalog network calls through
  GitHub-auth-aware opener
- Adds comprehensive unit/integration tests for auth header behavior
- Updates user docs for both extensions and presets

Co-authored-by: anasseth <16745089+anasseth@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix(auth): address review feedback from #2087

- Fix redirect handler to preserve Authorization on GitHub-to-GitHub
  redirects (e.g. github.com → codeload.github.com). The previous
  implementation relied on super().redirect_request() which strips
  auth on cross-host redirects, breaking private repo archive downloads.
- Add codeload.github.com to documented host lists in both
  EXTENSION-USER-GUIDE.md and presets/README.md
- Add redirect auth-preservation and auth-stripping tests

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix(auth): use Bearer scheme instead of token for consistency

Aligns with the rest of the codebase (e.g. __init__.py:1721) and
GitHub's current API guidance. Updates all test assertions accordingly.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: address second round of Copilot review feedback

- Fix docstring to say Bearer instead of token (matches implementation)
- Remove unused imports/fixtures from redirect tests (GITHUB_HOSTS,
  MagicMock, temp_dir, monkeypatch)
- Replace __import__('io').BytesIO() with normal import io pattern
  in test_presets.py

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: anasseth <16745089+anasseth@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>