Add experimental.chat condition to chat session menu contributions

[Copilot]

The "Refresh Chat Sessions" command and related chat session menu items remain visible when githubPullRequests.experimental.chat is false.

Changes

• Added config.githubPullRequests.experimental.chat condition to chat/chatSessions menu items (pr.openChanges, pr.checkoutChatSessionPullRequest)
• Added same condition to chat/input/editing/sessionToolbar menu items (pr.checkoutFromDescription, pr.applyChangesFromDescription)

This aligns with how other AI features are gated (e.g., chatParticipants, languageModelTools).

{
  "command": "pr.openChanges",
  "when": "chatSessionType == copilot-cloud-agent && config.githubPullRequests.experimental.chat",
  "group": "inline"
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>GitHub Pull Requests: Refresh Chat Sessions command remains available with disableAIFeature set to true</issue_title>
<issue_description>

1. Set disableAIFeature setting to true.
2. Open command palette
3. Search for "Chat"

Expected result: You should only see one command (to enable AI)
Actual result: GitHub Pull Requests: Refresh Chat Sessions command is still available as well.
</issue_description>

<agent_instructions>All AI/Copilot features should be disabled when the disableAIFeatures setting is true. This includes the Copilot PR query, all sparkle actions, and any other AI/Copilot features.</agent_instructions>

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

• Fixes GitHub Pull Requests: Refresh Chat Sessions command remains available with disableAIFeature set to true #8376

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