Expose full chat history to chat session agents

[Copilot]

Chat session agents (registered via the chat session API) were not receiving full history metadata - they only received metadata for their own turns, not for the entire conversation.

Root Cause

The isContributedChatSession flag was passed from mainThreadChatAgents2.ts but didn't flow through the protocol to prepareHistoryTurns, which decides whether to include metadata:

// prepareHistoryTurns strips metadata unless conditions are met
const result: vscode.ChatResult = agentId === h.request.agentId || context.isContributedChatSession ?
    ehResult :
    { ...ehResult, metadata: undefined };  // metadata stripped here

Changes

• Protocol alignment: Moved isContributedChatSession from nested chatSummary object to root context level in ExtHostChatAgentsShape2 interface
• Flow completion: Updated $invokeAgent, $provideFollowups, $provideChatTitle, and $provideChatSummary to pass isContributedChatSession through their context objects
• Type propagation: Added isContributedChatSession to _createRequest signature to forward flag to prepareHistoryTurns

Chat session agents now receive full history metadata across all agent interactions (invoke, followups, title, summary).

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