Add isReadOnly capability to chat sessions

[Copilot]

This PR implements the isReadOnly capability for chat sessions, allowing extensions to specify whether a chat session should be in read-only mode. The capability defaults to false to maintain backward compatibility.

Changes

API Changes

• Added isReadOnly?: boolean property to the ChatSessionCapabilities interface in the proposed API
• The property is optional and defaults to false when not specified

Implementation

The capability flows through the system following the same pattern as existing capabilities like supportsInterruptions:

1. Extension Point Schema: Updated the chatSessions contribution point to accept the isReadOnly capability in package.json
2. Type Definitions: Added the property to all relevant interfaces and DTOs throughout the extension host system
3. Data Flow: The capability is passed from the extension's API through the extension host to the main thread

Usage Example

Extensions can now specify read-only chat sessions in their package.json:

{
  "contributes": {
    "chatSessions": [{
      "type": "myExtension.readOnlySession",
      "name": "@readonly",
      "displayName": "Read-only Session",
      "description": "A session that cannot accept new input",
      "capabilities": {
        "isReadOnly": true
      }
    }]
  }
}

Or programmatically when registering content providers:

vscode.chat.registerChatSessionContentProvider(
  'mySessionType', 
  provider, 
  { isReadOnly: true }
);

Testing

• Added comprehensive test coverage for the new capability
• Updated existing tests to handle the new property
• Validated TypeScript compilation across all modified files

The implementation follows VS Code's established patterns for chat session capabilities and maintains full backward compatibility.

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