Text renderer: materialize CharSequence values as String #130

[paulk-asert]

Fixes #130.

Text.renderCharSequence stored the raw CharSequence object in the DisplayData bundle. The value is later serialized by Gson, which handles String (and, by virtue of built-in adapters, StringBuilder/StringBuffer) but turns other implementations into a reflective field dump — or fails outright with JsonIOException on JDK-internal types such as CharBuffer under strong encapsulation.

The fix materializes the value once via String.valueOf(data) and serves that for every requested mime type.

New TextTest covers: plain String, StringBuilder, CharBuffer (the JsonIOException case), a custom CharSequence (stand-in for library types like commons-text TextStringBuilder or Groovy's GString, which is how this surfaced), and an alternate requested mime type. jjava-jupyter module: 151 tests green.