The same pattern for reading an error response body appeared in three
places across the auth handlers:
- `internal/api/handlers/v0/auth/github_at.go` (twice)
- `internal/api/handlers/v0/auth/github_oidc.go` (once)
A shared `readBody` helper function added to `common.go` now handles
this in one place.
The `io` import is removed from both files since it is no longer used
directly there.
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Co-authored-by: Radoslav Dimitrov <radoslav@stacklok.com>
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