IgniteUI / igniteui-angular / 31495034279

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fix(grid): restore trailing scroll emission so top rows aren't lost after fast scroll (#17455)

* fix(grid): ensure final settle position in momentum scroll with trailing option

* test(grid): add regression coverage for momentum-scroll trailing settle

Drives the vertical scrollNotify stream with an intermediate (leading-edge)
position followed by a scrollTop=0 settle in the same throttle window, so the
settle can only be delivered on the trailing edge. Asserts the grid resets to
startIndex 0 / renders row 0, guarding against the dropped-trailing throttle
config that left the top rows frozen out of view.

* test(grid): move throttle-trailing test to virtualization block, add bottom case

Address review feedback:
- Relocate out of the zoneless regressions block into "IgxGrid - virtualization
  tests"; the fix is in the RxJS throttle config and is zone-independent.
- Use a fixture that overflows horizontally (30 columns in a 600px grid) and
  assert the horizontal scrollbar is present, matching the reported reproduction
  conditions.
- Add a bottom-boundary case so the fast momentum scroll to the bottom is also
  covered (issue #17464).

* test(grid): make bottom-boundary settle assertion null-safe

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