fix: stop retrying artifact upload on non-retryable status codes

[thomhurst]

Description

Follow-up to discussion #6448 (GitHub Enterprise Server users waiting ~30s for the HTML report artifact upload to fail).

GHES doesn't implement the Actions Results artifact API that actions/upload-artifact@v4+ — and TUnit's in-process uploader — depend on, so CreateArtifact returns 404. That should fail immediately, but on netstandard2.0 it didn't:

// HttpRequestException has no StatusCode on netstandard2.0, so we guessed from the message
if (msg.Contains("401") || msg.Contains("403") || ...) return false;
return true;   // <-- 404 lands here

A 404 was therefore treated as retryable and burned the whole 5-attempt backoff schedule (3s → 4.5s → 6.75s → 10.1s, plus jitter ≈ 25-30s) before surfacing the identical error.

Changes

• Added ArtifactUploadException, which carries the HTTP status code on every target framework. EnsureSuccessAsync throws it instead of HttpRequestException, removing both the #if NET split and the string heuristic.
• IsRetryable(int?) is now an allowlist of genuinely transient codes: 408, 429, 500, 502, 503, 504 (408 is new). Everything else — 404, 401, 403, 400, 422 — fails on the first attempt.
• 404 responses include a hint that the artifact API is unavailable on this host, which is expected on GitHub Enterprise Server.
• Non-retryable failures still propagate out of UploadAsync, so the CreateArtifact name-dedup loop doesn't re-issue the same doomed request three times. HtmlReporter catches and warns exactly as before.

Complements #6447 rather than replacing it — that flag is still the way to skip the upload entirely; this makes the un-flagged path cost one request instead of five.

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Testing

14 new IsRetryable cases in TUnit.Engine.Tests/GitHubArtifactUploaderTests.cs covering the transient allowlist, the non-retryable codes (incl. the GHES 404), and the no-status case. 20/20 pass on net10.0.

No source generator or public API surface touched, so no snapshot updates required.