Bridge a Mini App page to an agent turn #260

Parent

#251

What to build

A Mini App page can run and answer an agent turn through the bridge, and can ask the host to show a
session it created.

Acceptance criteria

• The bridge's agent methods reach the runtime and return contract-shaped results and failures.
• A page's run streams its events to the page, and cancelling the run stops it.
• A permission request raised by that turn is answerable from the page, and the turn proceeds.
• A questionnaire raised by that turn is answerable from the page.
• A page's request to show a session selects that session in the client, reports it as shown while
  it is selected, and reports it closed when the person leaves it.
• A permission asked by such a session is denied while nothing shows it, and answerable while the
  session rail has it selected.
• A page that calls an agent method the host cannot serve receives a documented failure code rather
  than a hang.