Regenerate clearContext bindings for the tightened runtime contract

Mirrors github/copilot-agent-runtime#14002 after review:

- `HistoryClearContextRequest.prompt` is now required. A cleared window
  holding only system and developer messages is not a conversation a model
  can answer, so every clear seeds the window it creates.
- `HistoryClearContextResult` loses the `cleared` discriminator. The RPC now
  rejects the cases it was meant to describe - a remote session, or a call
  made while no tool call is in flight - so there is one error channel
  instead of a success flag plus an error channel.
- `ContextClearedData.prependMessages` is gone. It had no producer, and it
  was a permanent commitment on a durable event for a code path nothing
  exercised.

Regenerated with `scripts/codegen`; only the clear-context hunks are taken,
so unrelated schema drift stays out of this PR.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: examon