Truncate wide cells in the interactive table browser

[tanishgupta-db]

Changes

Truncates each cell to a fixed width (with an ellipsis) in the interactive table browser
(libs/tableview). Previously a single very large value — for example a big BLOB — made its column
extremely wide and degraded rendering and horizontal scrolling.

This affects all consumers of the interactive browser: experimental aitools tools query,
experimental postgres query, and the experimental preview-datasets command. Normal values still
show in full; only unusually long cells are shortened.

Why

The browser sized columns to the full length of the widest value with no upper bound, so one
oversized cell could make the table unusable. Bounding each cell keeps the layout stable regardless
of the data.

Tests

• Unit tests for the truncation helper (short, at-limit, over-limit, and multibyte inputs) and for
  the rendered table lines with an oversized cell.
• Existing libs/tableview, aitools, and postgres tests still pass.

[github-actions]

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Inputs:

• PR number: 6034
• Commit SHA: b0a68c2be4f463941cf9bf0732803545b6342435

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