[two_dimensional_scrollables] Fix pinned cells hit test

[beroso]

This fixes a bug where TableView trailing pinned cells lose hit tests to underlying cells.

List which issues are fixed by this PR. You must list at least one issue.
Fix flutter/flutter#186876

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Code Review

This pull request modifies RenderTableViewport to ensure hit testing correctly accounts for pinned areas by intersecting cell rectangles with section-specific clip rectangles. It introduces a _sectionClipRectFor method to calculate these bounds and adds regression tests to verify hit-test precedence for pinned cells. Review feedback suggests optimizing hit testing by using dual contains checks to avoid unnecessary object allocations and caching viewportDimension in _sectionClipRectFor to improve readability and performance.