* Match CPython error type for non-ASCII struct format arguments
Struct() raised the wrong exception type when the format argument
contained non-ASCII characters:
- str input with non-ASCII char: RustPython raised UnicodeDecodeError
with an empty message; CPython raises UnicodeEncodeError as if
format.encode('ascii') had been called directly.
- bytes input with non-ASCII byte: same wrong UnicodeDecodeError;
CPython passes the bytes through to the format parser, which then
errors with struct.error("bad char in struct format").
Restructure IntoStructFormatBytes::try_from_object to:
- raise UnicodeEncodeError("ascii", s, start, start+1, "ordinal not
in range(128)") for non-ASCII str, with start computed as the
first non-ASCII code point position (matching CPython's natural
encoding-error format);
- raise struct.error("bad char in struct format") for non-ASCII bytes,
produced via the existing new_struct_error helper.
Probed byte-identical with CPython 3.14.4 for both cases. Full
test.test_struct (43 tests) passes with no regressions. Sanity-tested
all standard format/pack/unpack/calcsize call shapes remain unchanged.
* Add regression test for non-ASCII format string error types
* Use raise AssertionError instead of assert False (B011)
