[Bug]: Short MCP App cards reserve excessive blank space in the mobile feed #17032

Describe the bug

Short MCP App cards leave large blank gaps in the native mobile thread feed. Each inline app reserves 420 points even when the app reports a much smaller content height. Two consecutive Confluence previews require most of the screen just to show their short summaries.

Reproduction

1. Import the sanitized MCP fixture into an isolated T3 environment.
2. Connect the native mobile client and open the settled MCP embed regression fixture.
3. Collapse the completed reasoning trace and scroll to the two consecutive Confluence cards.
4. Observe the large blank area between the summaries.

Reproduced on iPhone 17, iOS 26.5, with the source mobile client at d720210. The sizing path is shared by iOS and Android; Android has not been verified yet.

Expected behavior

Short inline apps shrink to their reported content height. Tall apps retain a capped inline area with internal scrolling; fullscreen apps continue to fill their screen.

Investigation

McpAppWebView.tsx reserves MCP_APP_ROW_HEIGHT = 420 and ignores onSizeChanged. It also advertises an exact inline container height. ThreadFeed.tsx supplies the same fixed size to LegendList, so changing only the WebView height would leave the feed layout incorrect. This is separate from the document theme defect (#16991) and Jira analytics cookie crash (#16992).

The reusable fixture uses fictional results and retains the captured vendor HTML/CSP without the original company conversation.