Fix repr(Table) for group-by expressions

[windiana42]

Summary

• Fixes repr(pdt.Table) crashing with TypeError when the table has a group_by applied.
• The issue was in get_head_tail(): calling summarize(num_rows=pdt.count()) on a grouped table produces the group-by columns plus num_rows, then export(pdt.Scalar) fails because it expects exactly one column.
• The fix ungroups the table at the start of get_head_tail before summarizing and exporting.

Closes #96

Reproducer

import pydiverse.transform as pdt
from pydiverse.transform import C

tbl = pdt.Table({"a": [1, 2, 3], "b": [4, 5, 6], "c": [7, 8, 9]})
grouped = tbl >> pdt.group_by(C.a, C.b)
repr(grouped)  # was: TypeError, now prints the table correctly

Test plan

• Verified the error reproduces on main
• Verified repr() works after the fix

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