[wsjwu58-cmd]

Pickling a TaskiqResult whose exception needs a pickle-safe fallback currently replaces error on the original result as well. For example, an exception class defined inside a function is replaced by an internal wrapper, so a subsequent raise_for_error() on the original result raises that wrapper.

Copy the model's field mapping before preparing the serialized exception. The serialized fallback stays unchanged. The regression checks the round-trip error message and verifies that the original result still raises the same exception object.

Validation: 36 focused result/serialization tests passed, along with Ruff and Black on the changed files and MyPy over taskiq and tests (144 files). The full prek run could not complete because Windows denied writing a hook dependency's temporary wheel; the full test suite and cross-platform CI matrix were not run.

Prepared with AI assistance; the patch was independently reviewed and checked locally.

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