[3.14] gh-135368: Fix mocks on dataclass specs with `instance=True` (GH-135421) (#135503)

gh-135368: Fix mocks on dataclass specs with `instance=True` (GH-135421)

* gh-135368: Fix mocks on dataclass specs with `instance=True`

* Extend dataclass mock_methods

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(cherry picked from commit c8319a3fea9ff7f9b49008be3b5d681112bbe7f3)

Co-authored-by: sobolevn <mail@sobolevn.me>
Co-authored-by: Marc Mueller <30130371+cdce8p@users.noreply.github.com>