fix: Do not show Group column for tree-tables

[mofojed]

• If a table has no grouped columns or only one grouped column, no point in adding an extra Group column for that situation as it is just duplicate information
• Fixes retain previous behaviour for tree-tables, it doesn't make sense as they are parent/child not groups #1831
• Fixes for the case where there is only one group, don't show an extra column #1853
• Added unit tests, and tested with the following snippet:

from deephaven import read_csv, agg
from deephaven.constants import NULL_INT
from deephaven import empty_table

_source = empty_table(100).update_view(["ID = i", "Parent = i == 0 ? NULL_INT : (int)(i / 4)"])
_insurance = read_csv("https://media.githubusercontent.com/media/deephaven/examples/main/Insurance/csv/insurance.csv")

agg_list = [agg.avg(cols=["bmi", "expenses"])]
small_by_list = ["region"]
by_list = ["region", "age"]

no_group_tree = _source.tree(id_col="ID", parent_col="Parent")
no_group_agg = _insurance.rollup(aggs=agg_list, by=small_by_list, include_constituents=True)

group_rollup_no_agg = _insurance.rollup(aggs=[], by=by_list, include_constituents=True)
group_rollup_agg = _insurance.rollup(aggs=agg_list, by=by_list, include_constituents=True)

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 46.12%. Comparing base (aee4047) to head (6ef233a).

Files	Patch %	Lines
packages/iris-grid/src/IrisGridTreeTableModel.ts	22.22%	14 Missing ⚠️
packages/iris-grid/src/IrisGridTestUtils.ts	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1851      +/-   ##
==========================================
+ Coverage   46.08%   46.12%   +0.04%     
==========================================
  Files         635      635              
  Lines       38013    38025      +12     
  Branches     9608     9612       +4     
==========================================
+ Hits        17517    17540      +23     
+ Misses      20443    20432      -11     
  Partials       53       53              

Flag	Coverage Δ
unit	46.12% <25.00%> (+0.04%)	⬆️

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[mattrunyon]

Still want Don to check the behavior is as expected

[dsmmcken]

Approved for functionality.