iconv codecs corrupt stateful encodings when decoding incrementally

Bug description:

The iconv-backed codecs start a fresh conversion on every call, so an incremental decode of a stateful encoding loses the shift state and silently returns wrong text. iso-2022-cn has no built-in codec, so the plain name reaches the iconv codec:

import codecs
blob = 'ABC中文DEF'.encode('iso-2022-cn')
print('one-shot:   ', blob.decode('iso-2022-cn'))
d = codecs.getincrementaldecoder('iso-2022-cn')()
print('incremental:', ''.join(d.decode(bytes([b])) for b in blob) + d.decode(b'', True))

one-shot:    ABC中文DEF
incremental: ABCVPNDDEF

codecs.iterdecode() and StreamReader.read(1) are wrong in the same way. A stateful encoding forced with the iconv: prefix is also affected, even when it has a built-in codec.

Either the codec objects need to keep one conversion across calls, or the iconv search function should refuse stateful encodings, which would make iso-2022-cn a LookupError.

The iconv codecs are new in 3.16 (gh-152997), so no released version is affected.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-154862

Reproduce the incremental-decoding example for the iconv-backed codec, then inspect the incremental decoder, codecs.iterdecode(), and StreamReader.read(1) paths described in the report. Done means stateful encodings preserve conversion state across calls, or the search function rejects unsupported stateful encodings consistently, with the shown cases covered by tests.