AddressHeader problem when FriendlyName is improperly quoted and contains comma

Bug report

Bug description:

Given an address text of the form: John Smith, Jr <johnsmith@example.com>
Because of the unquoted comma, the text is parsed as two addresses:
"John" <@smith>, "Jr" <johnsmith@example.com>
This is of greatest concern if the object is supposed to be a SingleAddressHeader.

I have also encountered TO and CC strings that have incomplete address specifications, containing only a Friendly Name. For these messages, I am only interested in the valid addresses, although others may want the existing behavior.

To work around both the parsing problem and the invalid address terms, I created a sub-class which discards any address where username or domain are not strings. My workaround does not correct for the incomplete Friendly Name, but it ensures a correct count of valid addresses. It would be desirable to implement a trap for unquoted commas, but there are so many challenges to parsing address lists correctly, I wonder if fixing the unquoted comma problem will create other headaches.

Problem observed on Python 3.11.3150.0 and Python 3.8.1

CPython versions tested on:

3.11

Operating systems tested on:

Windows

Start with the CPython email AddressHeader parsing behavior and reproduce the examples for an unquoted comma and an incomplete address term on the reported Python versions. Review the comment discussion before deciding the expected handling; done requires an agreed behavior for invalid or ambiguously parsed address entries.