fix(conversation): keep older-history pagination reliable during running turns

[turingcat]

Description

While a conversation turn is running, older message history can silently become unreachable, and the pagination/auto-scroll state around history loading can get corrupted:

1. Pagination never triggers when content doesn't overflow the viewport. A running turn folds consecutive tool calls into a single summary card and drops hidden/plan rows, so a full DB page can render as just a few short cards — too short to produce a scrollbar. Since the older-page loader only runs on onScroll, it never fires even though more history exists. MessageList.tsx now proactively pulls older pages (capped at MAX_VIEWPORT_FILL_PAGES) after mount/prepend as long as content still fits without a scrollbar.
2. Turn-completion reconciliation reset pagination progress. After a user scrolls up and loads older pages (oldestCursor/hasMoreBefore), a pending turn completing would reload the newest page and reset pagination as if it were a fresh mount, discarding that progress. loadMessages now accepts a preservePaginationBefore option, used by the turn-completion reconciliation path.
3. useAutoScroll misidentified prepended history as a new message. It compared only messages.length to detect "a new message arrived," but a prepended older page also grows the list length. It now compares the tail message by id instead.

Related Issues

• Closes [Bug]: Older message history fails to load in long-running conversations #4232

Type of Change

• fix — Bug fix (non-breaking change which fixes an issue)

Atomic PR Checklist (Rule 1)

• This PR contains exactly one feature or bug fix that cannot be further decomposed
• The PR title follows Conventional Commit format: <type>(<scope>): <subject> (English)

Local Checks (Rule 3)

• bun run format — formatting passes
• bun run lint — no lint errors (skip if no .ts/.tsx changed)
• bunx tsc --noEmit — no type errors (skip if no .ts/.tsx changed)
• bunx vitest run — tests pass (520 files / 4984 tests passed)
• i18n validated — N/A, no user-facing text changed
• New/changed user-facing text uses i18n keys (no hardcoded strings) — N/A, no user-facing text changed

Runtime Verification

• Verified on macOS
• Verified on Windows
• Verified on Linux
• I have performed a self-review of my own code

Additional Context

Regression tests added in tests/unit/renderer/messageList.dom.test.tsx, tests/unit/renderer/messageMerging.dom.test.tsx, and tests/unit/renderer/useAutoScroll.dom.test.tsx.