fix: render tool summary header and content as one container

[laileni-aws]

Problem

When a tool summary (for example an MCP tool call) is expanded in chat, the header (" ✓ Completed") and the expanded content ("Parameters" and results) render as separate rounded boxes. Per the UX spec they should read as a single container — the bottom of the header and the top of the content should not be rounded, with a divider between the header and the content.

Fix

In src/styles/components/chat/_chat-item-card.scss, when the summary is expanded (.show-summary):

• Square the header's bottom corners (border-bottom-*-radius: 0).
• Give the expanded content matching left/right/bottom borders and rounded bottom corners, and overflow: hidden so the inner parameter/result cards are clipped to the container shape.
• The header's existing bottom border acts as the divider between the header and the content.

The result is a single bordered container: header on top, a divider, then the parameters/results, with only the outer corners rounded.

Scope / testing

• Change is scoped entirely to the .mynah-chat-item-card-summary styles (the tool-summary component); no other components are affected.
• npm run build compiles the styles successfully; full unit suite and eslint/prettier pass.
• There are no end-to-end screenshot snapshots covering the tool-summary component, so no baselines are affected.
• This is a CSS-only presentation change. I was not able to render it in a live IDE from this environment, so a quick visual sign-off against the design is recommended before marking ready.

Additional change

• ui-tests/__test__/flows/quick-action-commands-header.ts: coerced a Playwright evaluate result to a boolean with Boolean(...) to satisfy @typescript-eslint/strict-boolean-expressions and keep the repo lint gate green.