.NET: Make “Start Chat” button clearly visible

[tecnocrata]

Motivation and Context

Previously, the “Start Chat” button sometimes disappeared after a conversation began, so users couldn’t reliably find the primary action. Disabling it also caused confusion because it looked clickable yet did nothing. The goal is to keep the button consistently visible (and not disabled) while ensuring clicks are safely ignored when a new chat shouldn’t start.

Description

• Always render the “Start Chat” button so the primary action remains discoverable.
• Keep the button enabled and rely on a guard inside StartNewConversation to no-op when starting a new chat isn’t allowed (e.g., no agent selected, conversation active, or streaming in progress), preventing unwanted invocations without hiding or disabling the control.
• Maintain the aligned visual state via the dedicated start-chat-btn-disabled styling without !important, using a more specific selector to show a muted look when appropriate.
• Leave the rest of the chat flow unchanged: conversations start as before, and the button remains available once conditions permit starting a new session.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR improves the user experience of the "Start Chat" button by ensuring it remains visible and clearly indicates when it cannot be used. Previously, the button would disappear when a session was active, causing confusion about the primary action.

Key Changes:

• The "Start Chat" button is now always rendered regardless of conversation state
• A new IsStartChatDisabled property centralizes the logic for determining when the button should be disabled
• New CSS styling visually distinguishes the disabled state with grayed-out colors and a not-allowed cursor