Use NEW_CHAT command to create fresh chat sessions for each issue action

[Copilot]

When invoking "Fix with Copilot" or "Summarize with Copilot" on multiple issues, all suggestions were appended to the same chat session, making navigation difficult as the conversation grew.

Changes

• Replace chatCommand() (which returns QUICK_CHAT_OPEN or OPEN_CHAT) with commands.NEW_CHAT in both issue.chatSuggestFix and issue.chatSummarizeIssue commands
• Pass prompt as { inputValue: prompt } parameter instead of bare string to pre-populate the new session
• Remove unused chatCommand import

Example

// Before: Reuses existing chat session
commands.executeCommand(chatCommandID, vscode.l10n.t('@githubpr Find a fix for issue {0}/{1}#{2}', ...));

// After: Creates new chat session each time
commands.executeCommand(commands.NEW_CHAT, { inputValue: vscode.l10n.t('@githubpr Find a fix for issue {0}/{1}#{2}', ...) });

Each invocation now starts a fresh chat session, keeping unrelated issue discussions separate.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Use a new quick chat for each request</issue_title> <issue_description>Testing microsoft/vscode-pull-request-github#6321

• select an issue and invoke 'Fix with Copilot'. Quick chat opens and proposes something
• select a different issue and invoke 'Fix with Copilot'. Quick chat adds that suggestion under the previous conversation.

As the two entries are not related to each other, I would prefer a new chat. With each request, the quick chat gets larger and difficult to navigate</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes microsoft/vscode-pull-request-github#6357

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