Cancel the session title request when a turn is cancelled

Summary

• Ctrl-C during fx ask now exits within a moment while the Gateway has not sent response headers. Before, fx sat out the background session title request's 15 s timeout first.
• session/cancel on an ACP session's first prompt now gets its reply at once instead of after the title request's timeout.
• A cancelled first turn keeps the title fx derives from the prompt, since its generated title request is dropped.
• A turn that is not cancelled still waits for its generated title, as before.