ReactiveX / RxPY / 29865117778

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fix(operators): accept concurrent.futures.Future wherever futures are accepted (#806)

Since the 3.0 rewrite every operator tested for futures with
`isinstance(x, asyncio.Future)`, so a `concurrent.futures.Future` fell
through to `from_(...)` and raised `TypeError: 'Future' object is not
iterable`. RxPY 2 duck-typed futures via `add_done_callback` and accepted
both flavours.

Restore that behaviour: add a public `reactivex.typing.AnyFuture[T]` alias
(`asyncio.Future[T] | concurrent.futures.Future[T]`) and an internal
`is_future()` TypeIs guard, and use them at all 16 isinstance sites --
flat_map, merge, amb, catch, exclusive, skip_until, switch_latest,
take_until, timeout, while_do, case, defer, if_then, zip,
on_error_resume_next and from_future.

`from_future()` already worked with both kinds at runtime; only its
annotation said otherwise. `to_future()` keeps returning an
`asyncio.Future`, since it constructs one.

Also widen the annotations so the pattern type-checks: `flat_map` gains
future-returning overloads, `merge_all` accepts futures as inner sources,
and the `amb`/`take_until`/`timeout` facades now match their impls.

examples/parallel/timer.py works again as written, and gains the
`__main__` guard that ProcessPoolExecutor needs on non-fork start methods.

Fixes #709

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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