[two_dimensional_scrollables] Fix crash when trailing pinned spans fall within the cache extent

[icartsh]

RenderTableViewport._updateFirstAndLastVisibleCell binary searches for the last non-pinned span whose trailing offset reaches the target trailing pixel. When the whole table fits inside the viewport plus the cache extent that search finds nothing, and the code falls back to the last index of the metrics map:

if (_firstNonPinnedColumn != null) {
  _lastNonPinnedColumn ??= _columnMetrics.length - 1;
}

With trailingPinnedColumnCount (or trailingPinnedRowCount) greater than zero, that index is a pinned span. layoutChildSequence then asks for the same vicinity twice: once for the non-pinned quadrant and once for the trailing pinned quadrant.

The first layout hides the problem, because _needsDelegateRebuild routes both calls through _buildChild. Any later layout without a delegate rebuild — a scroll, for instance — takes the _reuseChild path, and the second request trips the assert in two_dimensional_viewport.dart:

'elementToReuse != null': Expected to re-use an element at (row: 0, column: 2), but none was found.

This is easy to hit in practice: a table with a pinned trailing "actions" or "total" column whose columns happen to fit the viewport crashes on the first scroll.

The fix excludes the trailing pinned spans from the fallback, matching what _updateColumnMetrics / _updateRowMetrics already do when they compute the same value (delegate.columnCount! - delegate.trailingPinnedColumnCount - 1). trailingPinned*Count is required to be 0 when the corresponding count is null, so the subtraction is a no-op for infinite tables.

Two regression tests are added, one for columns and one for rows. Both fail on main with the assert above and pass with the fix.

Fixes flutter/flutter#192152

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AI disclosure

Per the AI contribution guidelines: this change was written with the assistance of an AI coding agent (Claude Code). I have reviewed the diff, reproduced the crash against main, and verified the tests fail without the fix and pass with it. I take responsibility for the contents of this PR.

flutter test in packages/two_dimensional_scrollables:

• with the fix: +167: All tests passed!
• with lib/ reverted to main: the two new tests fail with 'elementToReuse != null': Expected to re-use an element at (row: 0, column: 2), but none was found.

🤖 Generated with Claude Code

https://claude.ai/code/session_01MRZHkuG9nCwvNdMgxU8gQm

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[gemini-code-assist]

Code Review

This pull request updates the two_dimensional_scrollables package to version 0.5.5. It fixes a crash occurring when trailing pinned rows or columns fall within the viewport's cache extent by excluding them from the calculation of the last non-pinned column and row indices in RenderTableViewport. This prevents the same columns or rows from being laid out in both the non-pinned and trailing pinned quadrants. Additionally, regression tests have been added to verify this behavior. There are no review comments to address.

[icartsh]

CLA signed.

[icartsh]

Dropped the co-author trailer so the CLA check only sees the signed author.