Correct iterator adaptor Chain

The iterator protocol specifies that the iteration ends with the return
value `None` from `.next()` (or `.next_back()`) and it is unspecified
what further calls return. The chain adaptor must account for this in
its DoubleEndedIterator implementation.

It uses three states:

- Both `a` and `b` are valid
- Only the Front iterator (`a`) is valid
- Only the Back iterator (`b`) is valid

The fourth state (neither iterator is valid) only occurs after Chain has
returned None once, so we don't need to store this state.

Fixes #26316

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