Adds POST /v1/runs/{run_id}/steer and bridges Browser-Extension/WebUI
session chat streams into the active run registry so live runs on those
surfaces are steerable too.
- steer accepted only while run status is exactly 'running'; stop/stopping/
terminal states return 409 run_not_accepting_steer even while cooperative
shutdown retains the agent reference
- session SSE disconnect/cancellation interrupts and drains the executor-
backed run instead of cancelling only the async wrapper; control refs stay
registered until the turn actually exits
- undelivered steer text (accepted after the final response) is preserved as
pending_steer on the terminal run.completed event/status so clients can
replay it as the next user turn instead of losing it
- docs for the endpoint, next-tool-boundary delivery, acceptance-vs-delivery
semantics
Salvaged from PR #54466 by @abundantbeing.
001bcb908e