chat: avoid session switch height flicker (#319227)

* chat: avoid session switch height flicker

Only schedule first-measurement chat row height updates when the measured row would otherwise exceed the list's allocated height. This preserves the cutoff fix from PR 318385 without reintroducing first-measurement relayouts for rows that already fit. (Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* chat: handle fractional row height allocations

Compare initial chat row measurements against the actual allocated height instead of the ceiled allocation so fractional row heights still schedule a corrective update when they would clip content. (Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>