## Why
`master` does not compile. Three PRs that each built individually combined badly:
- #92 changed `fetchBearerToken` to return `(string, time.Duration, error)` and added `cachedBearerToken` (which hashes the challenge via `sha256Hash`).
- #94 (streaming) removed the now-unused-in-that-PR `sha256Hash` helper and its `crypto/sha256` / `encoding/hex` imports.
- #89 (HEAD) added `headUpstream`, which calls `fetchBearerToken` expecting two return values.
Result on `master`: `internal/proxy/engine.go` fails to build (`assignment mismatch: 2 variables but fetchBearerToken returns 3 values`; `undefined: sha256Hash`).
## Changes
- Re-add the `sha256Hash` helper and its `crypto/sha256` + `encoding/hex` imports.
- Fix the `headUpstream` 401 path to handle `fetchBearerToken`s three return values.
## Validation
- `go build ./...`, `go vet`, and `make e2e` all pass.
Should merge before the other in-flight branches so they rebase onto a compiling `master`.
Reviewed-on: #96
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>
unkinben
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