Fix stray NUL bytes in codec_test.dart string literals

Two string literals contained a raw 0x00 byte where a space was intended. Stable Dart tolerated it, but the analyzer on the declared minimum SDK (3.5.0) rejected it as an illegal character, failing the CI compatibility leg. Replaced with explicit escapes; the fuse test keeps a deliberate zero byte to exercise COBS zero-stripping. Library sources were unaffected.

Signed-off-by: Alexander Salas Bastidas <ajsb85@firechip.dev>

Author: ajsb85