Improve KeywordList parsing to support escaped characters and nested structures

[krishnagjsForGit]

Closes #12810

What I changed Implemented escape handling in KeywordList.parse(String, Character, Character) to support escaping the keyword separator using backslash (). This prevents incorrect splitting of keywords when delimiters appear within the keyword itself.

Refactored the parsing logic for clarity, including renaming loop variables for better readability and intent.

Added test cases in KeywordListTest to cover escape scenarios such as:

Escaped delimiter: "one\,two" → ["one,two"]

Escaped backslash: "one\\two" → ["one\two"]

Mixed escaped and unescaped delimiters.

Where the changes are KeywordList.java: Modified the parse method to include escape handling logic using a character-by-character loop.

KeywordListTest.java: Added JUnit test cases to ensure parsing behaves correctly with escaped delimiters and backslashes.

Why I made these changes Fixes issue #12810: Current parsing breaks when delimiters appear inside keywords (e.g., in MeSH terms). There was no way to escape the delimiter character, leading to incorrect keyword splitting.

Improves consistency: Provides a more robust, predictable behavior for keyword parsing, especially when users or importers use delimiters in keyword values.

Next Steps Awaiting review and feedback.

If the community prefers a different escape character or behavior, I’m happy to adjust the implementation.

After this is merged, similar logic could be extracted or reused where keyword parsing happens elsewhere (e.g., importers).

Mandatory checks

• [x] I own the copyright of the code submitted and I license it under the MIT license
• [/] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• [x] Tests created for changes (if applicable)
• [x] Manually tested changed features in running JabRef (always required)
• [/] Screenshots added in PR description (if change is visible to the user)
• [/] Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• [x] Checked 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 to the documentation repository.