test(translate): add hermetic unit tests for lang resolution, preflight, and oneshot request/decode

[Bonny07]

What

The translate package had no tests — this adds the first suite. Everything is hermetic: callOneshot is exercised against httptest, so go test ./... stays fast and offline (no live DeepL calls), and the existing CI test step covers it with no workflow change.

Coverage

• resolveTargetLang / resolveSourceLang — case-insensitive normalization (zh-hans → zh-Hans), the EN/PT target-vs-source asymmetry (EN → en-US as a target but en as a source), auto handling on each side, and the "unsupported code" errors.
• TranslateByDeepLX preflight — empty text → 404, invalid language → 400, and over-maxFreeTextLength → 413, including a multibyte case that proves the cap counts runes, not bytes. Only the pre-network validation paths are exercised, so no upstream request is made.
• callOneshot — request shaping (Authorization: None vs Bearer <token>, Content-Type / Origin / Sec-Fetch-*, body pass-through) and the three hand-rolled decode paths (gzip / deflate / br) plus identity.
• newInstanceID — RFC 4122 v4 shape, and that the process-wide instanceID is valid.
• oneshotRequest JSON — field order + source_lang omitempty + os_version tracking the pinned Chrome major, locking the byte-identical body contract.

Notes

• Pure test addition — no production code changed.
• Tests intentionally lock in current shipped behavior (e.g. empty text → 404). Where that behavior looks surprising I've left it as-is here and would rather open a separate issue to discuss than fold a behavior change into a test PR.
• Pairs with ci: drop dead golint step, align Go to go.mod 1.25, bump checkout to v4 #228 (CI cleanup): with tests in place, adding go vet / gofmt gates to CI becomes meaningful.

Ran locally: go test ./translate/ green (36 subtests), plus gofmt, go vet, and go build ./... clean.