Fix occasional ClassCastException when parsing Maven pom.xml files

[Achal1607]

When you open any Maven project, when Netbeans tries to parse pom.xml sometimes it shows the class cast exception as seen in issue oracle/javavscode#378 and oracle/javavscode#185. The root cause is if some of the modules are not included like org.netbeans.modules.editor.structure and org.netbeans.modules.editor.deprecated.pre65formatting, then LspMavenErrorProvider class sends document as FilterDocument instead of NbEditorDocument which causes this class cast exception. Since, FilterDocument extends StyledDocument and not LineDocument which is required by the BaseDocument.

This PR addresses above mentioned issue by introducing a new utility method that accepts a Document and ensures it is returned as a BaseDocument. If the given Document is already an instance of BaseDocument, it is returned as-is; otherwise, it is converted accordingly.
This approach ensures that the implementation remains agnostic to the available modules, providing a consistent and reliable way to work with BaseDocument regardless of the module configuration.

[mbien]

feel free to force push right away since the changeset is small.

[Achal1607]

Hey @mbien, Can you please review the PR?

[lahodaj]

The comment on line 79 talks about a possible deadlock, referring to this issue:
https://bz.apache.org/netbeans/show_bug.cgi?id=236116

It is not completely clear if the deadlock can still happen. But some care is probably needed here.

I wonder if the requirement for BaseDocument still applies - what would break if simply an ordinary Document would be used to create the ModelSource?

Alternatively, if the BaseDocument is really needed by the users of ModelSource, maybe the Document can be wrapped to BaseDocument in Utilities.createModelSource around line 187 using the approach from Utilities.getDocument? (Although the code to create the BaseDocument out of thin air in Utilities.getDocument seems dubious to me.) Possibly factor the BaseDocument creation out of the getDocument method, or something like that.