Improve URLDownload exception to include httpResponse details

[vivek1833]

Issue

Fixes #14089

Changes

This PR refactors DoiFetcher.performSearchById to use BibtexParser.parseEntries() instead of BibtexParser.singleFromString() and makes the asString(Charset, URLConnection) method private in URLDownload.

Changes Made:

1. DoiFetcher.java:

   • Updated performSearchById() to use BibtexParser.parseEntries() instead of singleFromString()

2. URLDownload.java:

   • Made asString(Charset encoding, URLConnection connection) method private

Testing:

• Code compiles successfully
• Existing functionality preserved

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• I added JUnit tests for changes (if applicable)
• I added screenshots in the PR description (if change is visible to the user)
• I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• I checked the user documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.

[koppor]

There is no URLconnection passed - please use org.jabref.logic.importer.fileformat.BibtexParser#parseEntries

[vivek1833]

Thanks for the review. The BibtexParser class implements the Parser interface, which provides parseEntries(String) as a default method that internally converts the String to an InputStream and calls the parseEntries(InputStream) method that BibtexParser implements. There is no parseEntries(URLConnection) method available in our API. This change replaces the static singleFromString call with the BibtexParser#parseEntries.

[koppor]

I made a typo and meant "stream". Could be seen at the issue description, but maybe for a newcomer, this is too hard to understand.

[koppor]

To be able to properly guide, I started to work on the issue for myself. The issue was a bit larger than I thought. And finally, I solved it for myself at #14384.

Please search for another issue.