[pganssle]

Stacked on #159770; only the top two commits are new here.

When an encoded word does not fit in the remaining space of a line, _fold_as_ew shrinks the chunk being encoded one character at a time until it fits. Nothing stops it from shrinking the chunk to nothing, and an empty chunk makes no progress: the loop appends an empty encoded word and a new continuation line forever.

>>> import email.policy
>>> email.policy.default.clone(max_line_length=20).fold('Subject', '😀')
# never returns

This is reachable whenever a single character's encoded form is wider than a fresh continuation line, e.g. any character outside the BMP at max_line_length=20 (the q-encoded word is 24 characters). The two earlier fixes in this area (bpo-33529, bpo-36564) handled lines too short for the RFC 2047 chrome, but not lines too short for the chrome plus one character.

The fix keeps at least one character in the chunk, so the loop always advances and the line runs over max_line_length instead. That matches what _refold_parse_tree already does when it cannot fold a part at all, and avoids raising for widths that work for every other input.

Tests, next to the two earlier infinite-loop regressions in test_policy.py:

• An exact-output regression case, as the earlier two have.
• A Hypothesis property: folding any generated unstructured value at any width in 1..120 (plus 0 and None) either raises HeaderParseError (only possible at widths of 13 or fewer, where no encoded word fits) or reparses to the same value modulo leading whitespace, which the parser drops. 100 examples take about 0.2 s; the explicit examples replay under the stubs.
• A separate expectedFailure test for a second finding: a header value that is itself a well-formed encoded word is written verbatim, so reparsing decodes =?utf-8?q?x?= to x. As far as I can tell RFC 2047 §5 requires such text to be encoded. Recorded rather than worked around, so it starts failing when fixed; a separate issue for it is coming.

• Issue: [Meta] Research: what can we test with Hypothesis? #107862