[QTI] Assessment tab does not fill full available height in channelEdit page #5938

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Context

This issue was discovered as a result of the assessment editor visual refactor in #5924. It is out of scope for that PR and should be addressed as a follow-up — the fix should improve layout handling in the channelEdit page so the assessment tab takes the full available height.

Observed behavior

The assessment items tab area does not fill the full available height of the channelEdit page. When there is only one question and it is collapsed, the content area ends after the last element, leaving an incorrect white background visible below instead of the expected gray background.

Expected behavior

The assessment tab content area should fill the full available height of the channelEdit page, with the gray background extending to the bottom regardless of how many questions are present or whether they are expanded or collapsed.

User-facing consequences

The exercise editor looks visually broken when there are few questions — the white gap below the content makes the page appear unfinished or incorrectly styled.

Steps to reproduce

1. Open or create an exercise node in Studio.
2. Navigate to the Questions tab.
3. Ensure there is only one question and it is collapsed.
4. Observe that the gray background does not extend to the bottom of the page — a white area is visible below the content.

AI usage

Used Claude to help draft this issue from a description and a screenshot. Reviewed and confirmed all sections for accuracy.