Fix infinite loop in email.message.EmailMessage.as_string() with long parameter keys

The infinite loop occurred in _fold_mime_parameters() when processing MIME parameters
with very long keys (64 characters) during RFC 2231 encoding. The issue was in
two locations:

1. In email._header_value_parser._fold_mime_parameters():
   - Replace infinite 'while True:' loop with 'while splitpoint > 1:'
   - Ensure splitpoint is always at least 1 to prevent getting stuck
   - Add fallback logic to force minimal splits when values cannot fit

2. In email.header._ValueFormatter._append_chunk():
   - Add safety check for extremely long strings that cannot be split
   - Force line breaks when no suitable split points are found
   - Prevent infinite loops in header folding for edge cases

This fixes GitHub issue #138223 where add_attachment() with long parameter
keys would cause as_string() to hang indefinitely during MIME parameter
folding and header processing.

Author: ChenyangLi4288

Misc/NEWS.d/next/Library/2025-01-27-12-00-00.gh-issue-138223.email-infinite-loop.rst

@@ -0,0 +1 @@
+Fix infinite loop in email.message.EmailMessage.as_string() when add_attachment() is called with very long parameter keys.