Fixes assertion failure in Unicode escape decoding when using custom error handlers that return single-character replacements and rewind input position.

Root Cause

The issue occurs in unicode_decode_call_errorhandler_writer() when:

Error handler returns a replacement string of length 1 (replen == 1)
Error handler rewinds input position (increasing remaining input bytes)
Original code failed to account for replacement length in buffer calculations

This led to assertion failures in debug builds.

Fix

Always include full replacement length in writer.min_length calculations
Change condition from replen > 1 to replen > 0
Add full replen instead of replen - 1 to maintain buffer invariant

Testing

Reproduction case now works without assertion failure
All codec callback tests pass (43/43)
Specific mutating decode handler test passes
No regressions detected in broader codec tests

Fixes#141336

Issue:Assertion failure in Objects/unicodeobject.c_PyUnicode_DecodeUnicodeEscapeInternal2: Assertion 'end - s <= writer.size - writer.pos' failed #141336

pythongh-141336: Fix Unicode escape assertion failure in error handler

4f20106

When unicode_decode_call_errorhandler_writer() processes a replacementstring of length 1 and the error handler rewinds the input position(increasing remaining input), the original code failed to account forthe replacement length in writer.min_length calculations.This resulted in assertion failures when writer capacity was insufficientafter writing the replacement: assert(end - s <= writer.size - writer.pos).The fix ensures the full replacement length is always included inwriter.min_length, regardless of replacement size, maintaining thebuffer capacity invariant for all decoders using this error helper.