Fix Document.isText() media type handling and add test coverage

[karansahani78]

Improve handling of media types in Document.isText()

Previously, certain text-based media types were not correctly identified as text.
This change improves the detection logic to properly handle such cases.

Additionally, a new test class (DocumentIsTextMediaTest) has been added to
verify correct behavior across different media types.

• Fix media type detection in Document.isText()
• Add test coverage for text and non-text media types

[karansahani78]

This PR is related to #5600.

Based on the ongoing discussion there, it seems introducing a clearer distinction for “textual media” may be preferred over modifying isText() directly.

I’m keeping this PR as a reference for the current approach, but I’m happy to refactor it (e.g. introduce a helper like isTextualMedia() or adjust toward a formatter-based solution) once the direction is confirmed.