test(datagrid): isolate header title-rect padding test from the value-filter funnel

[datlechin]

Problem

macOS App Tests has been red on main since #1756, failing only SortableHeaderCellTests:

"Title rect uses data cell horizontal padding" — (titleRect.width → 69.0) == (92 → 92.0)

Every PR branched off main inherits the red job (e.g. #1758, #1760), even when its own changes are unrelated and its other test jobs pass.

Root cause

#1756 added the per-column value filter, giving SortableHeaderCell a funnel and a default supportsValueFilter = true. titleRect(forBounds:) now subtracts reservedTrailingWidth(), which reserves the funnel width (13) + spacing (2) + padding (8) = 23pt when the filter is supported. So for a 100pt-wide bounds the title rect is 100 − 8 − 23 = 69, not 92.

The test builds a default cell (funnel on) but still asserts the old no-funnel width of 92, so it fails deterministically. Real data columns keep supportsValueFilter = true; only the row-number column sets it false.

Fix

The test's purpose is the horizontal padding (the left inset and the minX == 14 / width assertions), not the funnel, so it now sets supportsValueFilter = false to isolate that. Width is 100 − 8 = 92 again. Funnel and indicator reservation are already covered by the sibling tests in the same suite.

Test-only change, no user-facing behavior, so no CHANGELOG entry.

https://claude.ai/code/session_0198faM6VCrViRU4XwRoS1DC