[two_dimensional_scrollables] Fix repainted trailing pinned cells

[Piinks]

This PR fixes a performance bug in TableView where spans designated as trailing pinned were being incorrectly included in the range of "regular" (non-pinned) spans.

When the trailing pinned spans feature was introduced, these spans were appended to the end of the internal metrics maps. The logic in RenderTableViewport._updateFirstAndLastVisibleCell was defaulting the "last visible non-pinned index" to the very end of these maps (metrics.length - 1) whenever the binary search didn't find a clear boundary (e.g., when regular spans didn't fill the viewport).

This caused trailing pinned spans to be included in both the regular layout pass and the pinned layout pass. While visually hidden by clipping, this resulted in unnecessary iterations, building, and laying out of cells.

Updated _updateFirstAndLastVisibleCell in lib/src/table_view/table.dart to correctly cap the non-pinned range using the pre-calculated _lastRegularColumnIndex and _lastRegularRowIndex instead of the map length. This ensures the regular layout pass strictly terminates before reaching any trailing pinned spans.

Fixes flutter/flutter#185842

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²