gh-99533: Fix issue where email.generator.Generator ignores policy when using multipart/signed

[jackpendley]

Provides fix for Issue #99533

The requested fix in the original issue report was implemented.

Removed _handle_multipart_signed from generator.py altogether so the headers folding no longer changes when accessing the whole message at once

• Issue: email.generator.Generator ignores policy when using multipart/signed → ruins signing #99533

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[bitdancer]

Have things been fixed so that unmodified headers are never refolded? If that has been done, then this works. However, if it is still the case that long headers get refolded when rendering an existing (not a new) message, then this will break re-rendering existing signed messages, which was the point of the original fix. The correct fix, obviously, is to never refold header lines that have not been modified since the message was deserialized, which is what I should have done originally (hindsight is wonderful :)

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