Harden Cord append size accounting in MessageLite serialization (#26170)

Issue: total_size = size + output->size() could overflow on extreme input/state combinations.

Change: Introduced:
  --output_size = output->size();
  --guard: if (output_size > SIZE_MAX - size) { ... return false; }
  --safe compute: const size_t total_size = size + output_size;

Security impact: Prevents integer-wraparound in serialization size accumulation, improving robustness against resource-exhaustion style inputs.

Closes #26170

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/26170 from nidu-ninja:message-lite-cord-size-overflow 7529d3c5af2473226caedc9a3fd6152dc432ae9e
PiperOrigin-RevId: 919303834