Per [GitHub's documentation for VSCode's agent
mode](https://docs.github.com/en/copilot/how-tos/chat/asking-github-copilot-questions-in-your-ide#agent-mode),
a premium request is charged per user-submitted prompt. rather than per
individual request the agent makes to an LLM. This PR matches Zed's
functionality to VSCode's, accurately indicating to GitHub's API whether
a given request is initiated by the user or by an agent, allowing a user
to be metered only for prompts they send.

See also: #31068

Release Notes:

- Improve Copilot premium request tracking