gh-120946: Add MIME-Version when making EmailMessage multipart

[limadog9]

Multipart messages created with EmailMessage.make_related() or add_related() can omit the required MIME-Version header. Add the header when an EmailMessage is converted to any multipart subtype, preserving an existing header.

Add regressions for all three multipart constructors, the related-image case, existing headers, and the distinction from MIMEPart. Update existing header-count expectations for the added header.

Fixes #120946.

Validation:

• The new regressions reproduce four failures before the fix.
• Full email suite on a CPython debug build: 1,825 tests, 2 skipped.
• Ruff, NEWS lint, and patchcheck.

• Issue: multipart/related mail generated with EmailMessage() class lacks mandatory MIME-Version header #120946