Data Explorer: Visually distinguish column names with leading/trailing whitespace or empty string

[wesm]

Aims to address #3084, #3089. Related to #5652, we also cannot distinguish empty strings or strings with leading whitespace in column names both in the data grid headers and the summary panel.

This PR applies the same approach we used for data cells to format column names in the grid headers and the summary pane. The appearance is could potentially be a bit odd because we use non-fixed-width fonts.

e2e: @:data-explorer

[github-actions]

E2E Tests 🚀  ^?
This PR will run tests tagged with: @critical @data-explorer

[juliasilge]

This is a big improvement, and so much more clear! 🙌

• The leading/trailing whitespace treatment looks great and consistent for both R and Python.
• The empty string column names for Python look great.
• The empty string column names for R are still rendering like this:

Does there need to be a backend change for the R case? As far as I can tell, the column name is literally just an empty string so I'm not sure why it may be rendering different.

[wesm]

Does there need to be a backend change for the R case? As far as I can tell, the column name is literally just an empty string so I'm not sure why it may be rendering different.

Indeed -- Ark is explicitly formatting empty column names like that, so we can change it to return a length-0 string: https://github.com/posit-dev/ark/blob/a569d6ee15d85723aec5625620c0593a7a8476a7/crates/ark/src/data_explorer/r_data_explorer.rs#L583

[wesm]

I'll go ahead and merge this and then make a PR to adjust the empty column representation returned by Ark