Fixes#38226
## Summary
Add `chi_middleware.GetHead` as the first `BeforeRouting` middleware on
the API router. This makes every API `GET` endpoint automatically handle
`HEAD` requests, as required by RFC 9110 §9.3.2.
Previously, `HEAD` requests to endpoints like `GET
/repos/{owner}/{repo}/git/commits/{sha}` returned `405 Method Not
Allowed`.
The web router already used this same middleware (see
`routers/web/web.go:261`), so this aligns API behaviour with the web
router.
## Changes
- `routers/api/v1/api.go`: add `chi_middleware.GetHead` middleware to
the API router
- `tests/integration/api_repo_git_commits_test.go`: add
`TestAPIReposGitCommitsHEAD` verifying HEAD returns 200 on a valid ref
and 404 (not 405) on a missing ref
1c718da16c