[two_dimensional_scrollables] Fix mouse event loop when calling setState in TableSpan.onEnter (#11606)

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.

Fixes https://github.com/flutter/flutter/issues/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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