Adds support for the Disposable return type that 12 driver methods now
return as of 1.59.1, so users can use them as context managers:

    async with await page.route("**/*", handler):
        ...
    # handler is automatically unrouted on exit

Affected methods (Page & BrowserContext): add_init_script,
expose_binding, expose_function, route. Plus Tracing.group and
Screencast.{start,show_overlay,show_actions}.

Two flavors:
- Disposable(ChannelOwner) wraps a driver-side Disposable channel
  (used where the protocol returns one, e.g. addInitScript).
- DisposableStub is a Python-side wrapper that calls a dispose lambda
  on exit (used for client-side pairings like route/unroute and
  tracing.group/group_end).

Codegen rewrites both classes to AsyncContextManager/SyncContextManager
in the generated public API so consumers see standard Python types.

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