chore: remove unused open_github_url/_StripAuthOnRedirect from _github_http.py

[jawwad-ali]

Closes #2876

Description

Removes the two helpers in src/specify_cli/_github_http.py that have no callers anywhere in the repo (production, tests, docs, templates, or workflows):

• open_github_url() — orphaned when feat: Config-driven opt-in authentication registry with multi-platform support #2393 moved download authentication to the config-driven registry in authentication/http.py. Its only remaining reference was its own definition.
• _StripAuthOnRedirect (the _github_http copy) — referenced only by open_github_url() itself, and a duplicate of the live redirect/strip-auth implementation in specify_cli/authentication/http.py (a separate class taking a hosts tuple, covered by tests/test_authentication.py::TestRedirectStripping), which is untouched.

No runtime behavior change — dead-code removal only. resolve_github_release_asset_api_url() (used by extensions.py, presets.py, and __init__.py) is retained with all 8 of its tests, and the module docstring is updated to describe what the module does today.

Caller-sweep evidence (string-literal, getattr, and __all__ dynamic patterns also checked — zero matches):

$ git grep -n "open_github_url"
src/specify_cli/_github_http.py:152:def open_github_url(...)    # definition only — no callers

$ git grep -n "_StripAuthOnRedirect"
src/specify_cli/_github_http.py:57,165                # dead copy (removed here)
src/specify_cli/authentication/http.py:59,138         # live implementation — untouched
tests/test_authentication.py:773,776,785,788,798,802  # all import the live authentication.http class

Decision on build_github_request() / GITHUB_HOSTS (acceptance criterion 3): kept as a tested, self-contained utility (11 passing tests in tests/test_github_http.py). That keeps this PR to a single source file with zero test-file changes. Happy to remove them here or in a follow-up if maintainers prefer.

Note on #2442: that open PR currently extends open_github_url, but #2876 was filed by #2442's own author after narrowing that work, asking for these symbols' removal — the corresponding hunks can be dropped from #2442 on its next rebase.

Testing

• Tested locally with uv run specify --help (exit 0)
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (N/A — the removed code has zero callers, so no command behavior changes)

Details (Windows 11, Python 3.12.12):

• uvx ruff check src/ — All checks passed
• uv run pytest tests/test_github_http.py tests/test_authentication.py -q — 100 passed, 1 skipped
• Full suite uv run pytest — 3474 passed, 160 skipped, 14 failed; counts identical to clean main on the same host. The 14 failures are pre-existing environment-only failures (os.symlink → WinError 1314; symlink creation requires elevation/Developer Mode on Windows), unrelated to this change and not touching _github_http.py.
• Repo-wide grep after the change: no references to the removed symbols remain.

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

This PR was developed with Claude Code (Claude Opus 4.8) under my direction — the caller-sweep analysis, the code removal, the local verification runs quoted above, and this description. I reviewed the diff and the evidence before submission. If any of my review responses are AI-assisted, I will disclose that in the thread as well.