Upstream cel-cpp module@0.14.0.envoy patches to source

### Module
- `cel-cpp`
- Version: 0.14.0.envoy
- BCR: ✅ exists (0.14.0)
- Source: https://github.com/google/cel-cpp

### Patches Applied
- `cel-cpp. patch`:
- Null pointer check in `byte_string.cc` (bug fix)
- `[[maybe_unused]]` attribute fixes in `typeinfo.h` (compiler warning)
- Type cast fix in `value.h` (signed/unsigned comparison)

### Checklist
- [ ] Upstream patches to google/cel-cpp (generic fixes)
- [ ] Once landed, BCR will pick up in next release
- [ ] Update envoy to use BCR version

### Notes
- Patches are generic bug/warning fixes - good candidates for upstreaming
- Patch: https://github.com/envoyproxy/toolshed/blob/main/bazel-registry/modules/cel-cpp/0.14.0.envoy/patches/cel-cpp.patch