[ty] Address intersection receiver review feedback (#25819)
## Summary
This follows up on [the post-merge
review](https://github.com/astral-sh/ruff/pull/25626#pullrequestreview-4466748160)
of #25626.
`functools.partial` now preserves the full intersection receiver when
delegating member lookup to its nominal `partial[T]` view. Bound-method
fallback still resets the receiver when lookup switches to the separate
underlying function object; the implementation and regression coverage
now make that distinction explicit.
This also parenthesizes compound bound-method receivers in displayed
types and clarifies and relocates the related intersection receiver
documentation and tests.
