Add fuzz targets for OBJ, PLY, STL, and animation decoders

[smoke-wolf]

Summary

Add four new fuzz harnesses to src/draco/tools/fuzz/ covering the I/O
format decoders and animation decoder that currently have no fuzz coverage:

• draco_obj_decoder_fuzzer.cc — OBJ text format parser
• draco_ply_decoder_fuzzer.cc — PLY binary/text format parser
• draco_stl_decoder_fuzzer.cc — STL binary/ASCII format parser
• draco_animation_decoder_fuzzer.cc — Keyframe animation decoder

Rationale

The existing four fuzz targets cover only the compressed .drc format
(draco::Decoder::DecodeMeshFromBuffer / DecodePointCloudFromBuffer).
The OBJ, PLY, and STL decoders accept untrusted file data from the same
sources (web, file imports) and have complex text/binary parsing logic
that would benefit from continuous fuzzing. The animation decoder shares
the compressed-format internal decompression path but through a separate
entry point that exercises keyframe-specific attribute handling.

Build integration

No CMake or build system changes needed — the existing
src/draco/tools/fuzz/build.sh (used by oss-fuzz) automatically
compiles all *.cc files in the fuzz directory. The new harnesses
follow the same pattern and include paths as the existing four.

Context

This was requested by the oss-fuzz maintainer (@DavidKorczynski) in
google/oss-fuzz#16096 — harnesses should live
upstream so oss-fuzz builds them from the main repository.

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[DavidKorczynski]

This was requested by the oss-fuzz maintainer (@DavidKorczynski) in
google/oss-fuzz#16096

To clarify: I did not request the creation of these fuzzing harnesses. However, as a maintainer of OSS-Fuzz I review PRs on that repo to make sure contributions align with how existing projects structure the fuzzing + oss-fuzz's preferences. To this end I request contributors place the fuzzing harnesses in the upstream projects, e.g. similar to what draco is already doing with its existing fuzzing harnesses.

I.e. my request is to "coordinate this with the Draco maintainers and make sure you have their collaboration".

[smoke-wolf]

Thanks for the clarification, I misworded that. What I meant was that you directed the contribution to the upstream repo rather than oss-fuzz, which makes sense. These harnesses cover the unfuzzed I/O format decoders (OBJ, PLY, STL) and animation decoder as a complement to the existing four .drc targets.