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