#22831 test(next): chat history with an oversized record at the very top

Follow-up to #22794 (coord-deputy asked for this case). It adds two tests to webviews/test/agent-pane-history-rowless-page.test.ts, covering an oversized record at the very start of a chat's history:

• The chat's first prompt carries a 600 KB paste, so the oldest page holds only that record. Scrolling to the top draws it with every later turn, and canLoadOlder ends false.
• A 600 KB prompt was queued and dropped before the first turn, so the oldest page draws no row. Scrolling to the top still ends: every ask moves the loader's key, canLoadOlder ends false, and both turns are there.

Tests only; no product change. The tier is low, but the repo has no risk:low label, so this PR carries risk:normal.

Changelog

none

Observed / Not run

• Ran scripts/web-lint.sh locally: it passed.
• scripts/ci/run-webviews-tests.sh on the fleet (step cb541c85ff4a51c0f24f58cf, aws-m4pro-7, head 8299b9b): PASS test/agent-pane-history-rowless-page.test.ts, all five tests including the two new ones. The run exits 1 on files this PR does not touch (agent-pane-column-edges, apps-flow, hover-row-sweep and others): most can't launch Playwright's chromium on that worker (Executable doesn't exist ... chrome-headless-shell).

Agent session: cosmic-gray

🤖 Generated with Claude Code

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Summary by cubic

Adds two tests to webviews/test/agent-pane-history-rowless-page.test.ts covering chat history when the oldest page holds an oversized record. The first test uses a 600 KB first prompt; the second uses a dropped oversized prompt that draws no row. Both verify that scrolling to the top ends with canLoadOlder set to false. Tests only, no product change.