[two_dimensional_scrollables] Fix mouse event loop when calling setState in TableSpan.onEnter

[Piinks]

This PR addresses a bug in TableView where calling setState (or any action that triggers a delegate rebuild) inside a TableSpan.onEnter callback would cause an infinite loop of onExit and onEnter events.

The RenderTableViewport was clearing its cached _columnMetrics and _rowMetrics (_Span objects) on every delegate rebuild. Since _Span implements MouseTrackerAnnotation, and the MouseTracker identifies regions by object identity, re-creating these objects every frame caused the MouseTracker to:

1. Trigger onExit for the old _Span object.
2. Trigger onEnter for the new _Span object.

If onEnter contained a setState call, this cycle would repeat indefinitely.

Fixesflutter/flutter#147614

Fix:

• Refactored _updateRowMetrics and _updateColumnMetrics to reuse existing _Span objects when
  available.
• Updated _Span.update to only refresh the internal configuration and layout metrics while
  maintaining the same object instance.
• Modified layoutChildSequence to avoid clearing the metrics maps, instead allowing the update
  methods to handle stale entries.
• Added logic to properly dispose and remove spans when the row or column count is reduced.

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[gemini-code-assist]

Code Review

This pull request addresses a bug where calling setState within onEnter in a TableSpan caused an infinite loop of onExit/onEnter events. The implementation optimizes RenderTableViewport by selectively disposing and removing column and row metrics when the count is reduced, rather than clearing all metrics during a layout rebuild. A regression test has been added to ensure the fix works as expected. The review feedback suggests refactoring the duplicated logic for disposing and removing trailing spans into private helper methods to improve code maintainability.

[AbdeMohlbi]

SGTM, with some nits

[AbdeMohlbi]

Fixes an infinite loop

[AbdeMohlbi]

I don't think that we usually name the test as regression test for issue #1234 but rather add a comment at the start of it.

[Piinks]

Good catch thank you! I will update