feat: Display custom single index column in anywidget mode

[shuoweil]

This PR introduces the single column display for anywidget mode.

A screenshot of single index column display is here: screen/6VLA4Nk68TsYczV

Fixes #<459515995> 🦕

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[tswast]

Would it be possible to check len(df.index.names) != 0 on the original BigFrames DataFrame instead? Most of the time the no-name index is going to be the default range index, but not always.

[tswast]

Actually, it looks like df.index.names won't work in partial ordering mode.

python-bigquery-dataframes/bigframes/dataframe.py

Line 287 in 481d938

@validations.requires_index

I'll try to figure out the correct alternative and report back.

[tswast]

df._block.has_index

[shuoweil]

Thanks again for the excellent feedback, @tswast. Your suggestion to use df._block.has_index was precisely what was needed to avoid the expensive index materialization on the full BigFrames DataFrame.

I've implemented the change to use self._dataframe._block.has_index. However, I found that an additional check was needed to fully resolve the issue and pass test_widget_with_default_index_should_display_row_column.

The full condition I've used is:
if self._dataframe._block.has_index and page_data.index.name is not None:

To clarify the inclusion of page_data.index.name is not None: this part of the check is performed on the small, local pandas page_data slice. It's a cheap operation because page_data is already in memory. Its purpose is to correctly handle the display formatting in all scenarios.

Specifically, while self._dataframe._block.has_index correctly tells us if a custom index exists on the BigFrames DataFrame, that index might still be an unnamed default (like a RangeIndex). In such cases, page_data.index.name would be None.

The combined check ensures that we only display a named custom index when both conditions are met:

1. A custom index has been defined on the BigFrames DataFrame (self._dataframe._block.has_index).
2. That index actually has a name (page_data.index.name is not None) to use as a header.

Otherwise, it correctly falls back to displaying the generic "Row" column. This makes the overall index display logic robust and accurate, reflecting the user's intent without triggering expensive BigQuery operations.

Let me know if this approach looks good to you.

[tswast]

Let's display the real index values, even if the index has no name.

Suggested change

	if self._dataframe._block.has_index and page_data.index.name is not None:
	# Custom named index - include it with its actual name
	page_data.insert(0, page_data.index.name, page_data.index)
	if self._dataframe._block.has_index:
	index_name = page_data.index.name
	page_data.insert(0, index_name if index_name is not None else "", page_data.index)

[shuoweil]

testcase is updated accordingly

[tswast]

Looks like this bug ID is incorrect. Maybe this is the correct one?

Suggested change

	# TODO(b/332316283): Add tests for custom multiindex
	# TODO(b/459515995): Add tests for custom multiindex

[shuoweil]

actually, I plan to use this one b/438181139

[tswast]

Are these changes intended? Maybe you missed my note about opting out of airlock in chat? I've also added instructions to our internal contributing docs.

[shuoweil]

I definitely should not include this noxfile change. I moidfy the noxfile file locally to run nox. I will revert this file change.

[tswast]

Bug ID looks incorrect here, too.

Suggested change

	# TODO(b/332316283): Add tests for custom multiindex
	# TODO(b/459515995): Add tests for custom multiindex

[shuoweil]

I would like to use b/438181139