Fix: URLSessionHTTPClient only move file if response code is within OK range

[otaviolima]

Only move the downloaded file if the HTTP status code is within the OK range

Motivation:

Our Package.swift has package dependencies that are hosted in our internal JFrog instance. We have noticed that when a server error occurs, a JSON payload is still returned. In our particular case the JFrog instance was returning HTTP 500 as the curl snippet below shows.

% curl --verbose -L \
  --netrc \
  --netrc-file bogusnetrc \
  -o "./Artifact.zip" \
  "https://host_redacted/Artifact.zip"
  <redacted logs>
 > GET /Artifact.zip HTTP/2
> Host: redacted_host
> Authorization: Basic <redacted>
> User-Agent: curl/8.7.1
> Accept: */*
> 
* Request completely sent off
< HTTP/2 500 
< date: Mon, 08 Dec 2025 11:01:50 GMT
< content-type: application/json;charset=ISO-8859-1
< content-length: 65
< 
% cat Artifact.zip 
{
  "errors" : [ {
    "status" : 500,
    "message" : ""
  } ]
}

Because of that, the error shown in the swift package PackagePath resolve is not very clear.

% swift package --package-path ./path/to/package resolve
Downloading binary artifact https://host_redacted/Artifact.zip
error: failed downloading 'https://host_redacted/Artifact.zip' which is required by binary target 'TargetName': $HOME_REDACTED/Library/Caches/org.swift.swiftpm/artifacts/ArtifactPath.zip already exists in file system
error: fatalError

This does not tell much about the exact underlying error that happened here.

Result:

After the changes in this PR, the error message will become much clearer about the underlying issue with the HTTP call

% ./build-folder/swift-package --package-path ./path/to/package resolve
Downloading binary artifact https://host_redacted/Artifact.zip
error: failed downloading 'https://host_redacted/Artifact.zip' which is required by binary target 'TargetName': badResponseStatusCode(500)
error: fatalError

[jakepetroules]

Instead of returning, perhaps this should throw so that the error handler below is consistently invoked for all types of failure?

[otaviolima]

Thanks for the suggestion @jakepetroules. I updated the PR following them.

Another change I made was to remove the lines that deleted the temporary file, based on the fact that the comment below states that the temporary file is cleaned up after delegate execution.

I tested again with my Package.swift and JFROG instance, and the error message is still the same one that I put in after changes in PR description, so from this side, the new code is yielding the same results as the initial implementation.