email.message.Message.add_header behavior differs from Message[header]=v

Bug report

Bug description:

Attempting to copy headers from an existing message to a new (to-be-cleaned copy of the original) message, I iterated over the old message, using something like

for h, v in oldmsg.items():
    newmsg.add_header(h, v)

which works for many of the test-messages in my test Maildir folder, but errors out on certain messages with

    newmsg.add_header(h, v)
    ~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/usr/local/lib/python3.13/email/message.py", line 587, in add_header
    self[_name] = SEMISPACE.join(parts)
                  ~~~~~~~~~~~~~~^^^^^^^
TypeError: sequence item 0: expected str instance, Header found

It appears that elements in msg.items() can produce a mixed-content list containing both (str, str) and (str, Header) elements; but add_header doesn't like receiving a Header object as a value, only str.

If I change it to use the .__setitem__() method function:

for h, v in oldmsg.items():
    newmsg[h] = v

it appears to work fine, appending the header that .items() gave me.

According to the .add_header() documentation, it should behave similar to the .__setitem__() method:

This method is similar to __setitem__() except that additional header parameters can be provided as keyword arguments. _name is the header field to add and _value is the primary value for the header.

The fix would involve making add_header accept the same sorts of Header values as __setitem__, or at least align the output of msg.items() with the types that msg.set_header() accepts. If not aligned, at a bare minimum, the documentation should reflect the difference in accepted value types (str vs Header)

This may also have tie-ins with similar Issues here:

• #65294
• #91957
• #101090

(manifested on 3.13 on OpenBSD, but it's standard-library stuff that would likely manifest on any OS)

CPython versions tested on:

3.13

Operating systems tested on:

Other

Linked PRs

• gh-151458

Review linked PR gh-151458 first, then inspect email/message.py around Message.add_header and compare it with Message.setitem and items(). Reproduce the mixed str/Header case from the report; done means the behavior is aligned or the accepted value-type difference is documented.