Fix: fallback conversion from BibTeX year field to biblatex date field

[wanling0000]

Closes #11868
This PR introduces a fallback mechanism in the "Convert to biblatex" cleanup step to improve compatibility with BibLaTeX-style date formats mistakenly entered in the BibTeX year field.

What was the problem?
Users reported that when year = {2011-11-11} (or other extended formats) is used in BibTeX mode, the "Convert to biblatex" cleanup did not populate the date field as expected.

This is because the cleanup logic only merges year + month if both are present and parsable. Non-standard values in year were silently ignored.

What has been changed?

• A fallback step was added after the main conversion logic:
  • If date is still missing, and
  • month is empty, and
  • year contains a parsable BibLaTeX-style date,
  • then date is set directly from year, and year is cleared.

This fallback is intentionally conservative and only runs if the standard cleanup logic does not produce a date.

Mandatory checks

• 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)
• Tests created for changes (if applicable)
• 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.
• 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.

[trag-bot]

The method applyDateFallback is new and should follow the Single-responsibility principle. It currently handles both date parsing and field clearing, which could be separated.

[trag-bot]

The method applyDateFallback should use withField instead of setField when creating a new BibEntry object to adhere to JabRef's coding standards.

[trag-bot]

@trag-bot didn't find any issues in the code! ✅✨