fix(ui): truncate Table column headers with ellipsis on overflow

Wrap column header text in a label span with text-overflow: ellipsis
so that long headers truncate instead of wrapping to multiple lines.

Signed-off-by: Johan Persson <johanopersson@gmail.com>

.changeset/fix-table-header-ellipsis.md

@@ -0,0 +1,7 @@
+---
+'@backstage/ui': patch
+---
+
+Fixed Table column headers overflowing and wrapping when there is not enough space. Headers now truncate with ellipsis instead.
+
+**Affected components:** Table