[JENKINS-72343] Accept all 2xx and 3xx status codes to validate proxy in HTTP Proxy Configuration

[lemeurherve]

The "Validate Proxy" button in "Manage > Plugins > Advanced settings > HTTP Proxy Configuration" currently accepts only HTTP_OK (200) as valid:

jenkins/core/src/main/java/hudson/ProxyConfiguration.java

Lines 579 to 580 in 51c5fa2

	if (httpResponse.statusCode() == HttpURLConnection.HTTP_OK) {
	return FormValidation.ok(Messages.ProxyConfiguration_Success());

This small PR allows accepting all status code below HTTP_BAD_REQUEST (400) as valid responses and add the status code in the response to the user:

See JENKINS-72343.

Testing done

Checked that it works by running Jenkins locally with mvn -pl war jetty:run.
Ran a build with tests locally with mvn -am -pl war,bom clean install.
There is no test for this client side functionality, I don't think one should be added.

Proposed changelog entries

• JENKINS-72343, Accept all 2xx and 3xx status codes to validate proxy in HTTP Proxy Configuration

(Not sure it worth being mentioned in changelog)

Proposed upgrade guidelines

N/A

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[timja]

seems reasonable, could maybe add the status code now given it's accepting more than 200?

[lemeurherve]

Added the status code in the response to the user:

[NotMyFault]

Thanks!

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/label ready-for-merge

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This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
Thanks!