Refine Duo Chat panel spacing and border radii

What does this MR do?

Refines the Duo Chat panel's visual rhythm so the input area, footer, and surrounding panel share a consistent spacing and border-radius scale, and removes a stale rule that produced a gap below the chat input.

Visual changes

• Chat input wrapper — replaces the asymmetric corner radii (gl-rounded-bl-[12px] gl-rounded-br-[18px] gl-rounded-tl-[12px] gl-rounded-tr-[12px]) with a uniform gl-rounded-lg, so the input echoes the surrounding panel geometry.
• Submit / cancel buttons — drops !gl-rounded-full so the buttons use the default Pajamas shape, and swaps the submit icon from paper-airplane to arrow-up to align with the visual direction Aura is moving toward.
• Footer — replaces the bespoke duo-chat-drawer-footer class with utility padding (gl-px-3 gl-pb-3) and nudges the disclaimer with gl-ml-2 so the footer aligns with the input.

Both chat views (duo_agentic_chat_view.vue and duo_chat_view.vue) receive the same treatment to keep them visually consistent.

Fix gap below the chat input

Removed a max-height rule on .ai-panel-body that used outdated SCSS variables and had a sign error, causing the panel body to be shorter than its flex container and leaving an empty strip below the chat input. The flex layout already sizes it correctly, so no replacement is needed.

References

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