Preserve wrapped converter return annotations in pipe

[Boulea7]

Summary

On early Python 3.11 releases, pipe() loses the return annotation of a final Converter because the instance signature cannot be inspected. Reading its annotated __call__ preserves the annotation without changing conversion behavior.

The regression passes on Python 3.11.2. The Python 3.14 suite and documentation checks pass, with combined Python 3.12/3.14 coverage at 100%. The full tox matrix has not been run locally.

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