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.

Changes made:
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._append_chunk():
   - Add comment explaining handling of extremely long strings that can't be split

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

Lib/email/_header_value_parser.py

@@ -3081,13 +3081,17 @@ def _fold_mime_parameters(part, lines, maxlen, encoding):
                 # the RFC standard width.
                 maxlen = 78
             splitpoint = maxchars = maxlen - chrome_len - 2
-            while True:
+            splitpoint = max(1, splitpoint)  # Ensure splitpoint is always at least 1
+            while splitpoint > 1:
                 partial = value[:splitpoint]
                 encoded_value = urllib.parse.quote(
                     partial, safe='', errors=error_handler)
                 if len(encoded_value) <= maxchars:
                     break
                 splitpoint -= 1
+            # If we still can't fit, force a minimal split
+            if splitpoint <= 1:
+                splitpoint = 1
             lines.append(" {}*{}*={}{}".format(
                 name, section, extra_chrome, encoded_value))
             extra_chrome = ''

Lib/email/header.py

@@ -527,6 +527,8 @@ def _append_chunk(self, fws, string):
                     continue
                 break
             else:
+                # No suitable split point found. Handle the case where we have
+                # an extremely long string that can't be split.
                 fws, part = self._current_line.pop()
                 if self._current_line._initial_size > 0:
                     # There will be a header, so leave it on a line by itself.