Avoid Windows ERROR macro collision in idl.h

[steadytao]

Summary

Avoid a Windows macro collision in flatbuffers/idl.h.

Problem

IDLOptions::ProtoIdGapAction currently defines an enumerator named ERROR.

On Windows, ERROR is commonly defined as a macro by system headers such as wingdi.h which causes flatbuffers/idl.h to fail to parse when both are included in the same translation unit.

Fix

• rename IDLOptions::ProtoIdGapAction::ERROR to ERROR_
• update the two internal references to that enumerator

Testing

• rebuilt flatc
• compiled a standalone translation unit with ERROR defined as a macro before including flatbuffers/idl.h
• confirmed the header now parses cleanly in that configuration

Fixes #8483.

[jtdavis777]

while this does fix the reported issue (and may be sufficient to merge as is), when I started to look into this there are several other enumerations in the flatbuffers headers which also have this collision. I went to fix them but I believe there are some that are speciified by grpc which felt problematic to change.

[steadytao]

I took a look and agree there are a few other ERROR enumerators with the same Windows macro-collision shape. I’m planning to expand this PR to cover the FlatBuffers-owned enums in public headers as well while avoiding gRPC-specified names since those feel riskier to rename.

[steadytao]

Ready for review.