chore: add Bazel 8 testing

[alexeagle]

The latest rules_proto has a fix to make this work, even though Bazel 8 moved the proto rule implementations to com_google_protobuf

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Changes are visible to end-users: no

Test plan

• Covered by existing test cases

[alexeagle]

Still two test failures. One of them is something trying to compile abseil-cpp from source, and we assert here that no functional CC toolchain should be required. I think it may be rules_python's fault though, rather than rules_proto or protobuf.

[fmeum]

Java protos still hit protocolbuffers/protobuf#19679, which hasn't been reviewed yet. But all other languages should work.

[alexeagle]

Thanks, I'll try removing Java as well.