tools: include MCP server definitions in debug export (#3022)

- Add vscode.proposed.mcpServerDefinitions proposed API declaration
- Enable mcpServerDefinitions in enabledApiProposals
- Add serializeMcpServers() helper function that:
  - Serializes stdio servers with command, args, cwd, env, and version
  - Serializes HTTP servers with masked uri, version (excludes Authorization header)
- Include mcpServers array in exportAllPromptLogsAsJson export output

This allows debugging chat sessions to include information about which MCP
servers were available during the session.

Fixes https://github.com/microsoft/vscode/issues/283945

(Commit message generated by Copilot)