Remove visual borders between chat containers

[Copilot]

Removes visual separators between agent-sessions-container and chat-controls-container in the chat view pane UI.

Changes:

• Removed border-bottom from stacked orientation
• Removed border-right/border-left from side-by-side orientations

All changes isolated to chatViewPane.css - no functional changes.

Original prompt

Remove the visual border that currently separates the chat-controls-container from the agent-sessions-container in the UI. Ensure that the containers appear seamlessly adjacent, without any border or visual separator between them.

Steps:

• Locate the UI code (likely CSS/SCSS/LESS) that defines styles for chat-controls-container and agent-sessions-container.
• Remove any border, box-shadow, or divider that visually separates these two containers.
• Test in the UI to verify that the transition between the two areas is seamless, with no unintended spacing or artifacts.

This pull request was created from Copilot chat.

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[bpasero]

I think for stacked, removing the border is fine:

But for side by side its odd because I need to find the sash to resize now: