Fix trailing null bytes in JSON output and add C-string termination

[olomix]

• Refactored buffer validation in groth16_prover_prove: Previously, buffer validation happened twice—first checking against minimum sizes before proof generation, then again against actual sizes after. This meant callers could
  receive PROVER_ERROR_SHORT_BUFFER twice: once for undersized buffers, then again after reallocating to minimum size if the actual proof was larger. Now validation occurs only after proof generation. Callers should use
  groth16_public_size_for_zkey_buf() or groth16_public_size_for_zkey_file() to pre-allocate minimum-sized buffers. If buffers are still insufficient after proof generation, the error message returns exact required sizes, allowing
  single reallocation.
• Improved error handling: Replaced strncpy() with snprintf()/vsnprintf() for safer string handling. Added CopyErrorFmt() with printf-style formatting and compile-time format checking. Input validation now uses early-return
  pattern instead of exceptions.
• Removed dead code: Deleted ShortBufferException, CheckAndUpdateBufferSizes(), and unused proofBufferMinSize()/publicBufferMinSize() methods.
• Documentation: Fixed PPOVER_ERROR → PROVER_ERROR typos. Added detailed parameter docs for groth16_prover_prove() clarifying in/out semantics and null terminator behavior.
• CI fix: Changed ctest to run from build_prover_macos_arm64 directory on macOS ARM64.