fix: stabilize CXX API generator on MacOS, make parser indempotent for nested enums

[artus9033]

Summary:

Fixes C++ API snapshot generation failures for nested enums and macOS/Linux output drift due to platform-dependent behavior, in the CXX API generator.

Problems

1. Duplicate enums: Doxygen parses codegen EventEmitters.h twice (direct codegen input + again via .mm includes). The parser raised RuntimeError: Identifier OnOrientationChangeOrientation already exists in scope ModalHostViewEventEmitter on ReactApple* views.

2. Inconsistent behaviour on MacOS vs Linux:

• for codegen component aliases (ConcreteComponentDescriptor, ConcreteViewShadowNode), macOS Doxygen emits hybrid XML definitions (typedef Type Name = Type) while Linux CI emits using Name = Type. The parser keyed off definition.startswith("typedef"), so identical source produced different .api output per platform.
• CASE_SENSE_NAMES = SYSTEM follows the host OS default (case-insensitive on macOS, case-sensitive on Linux), causing inconsistent symbol resolution.

Resolution

• .doxygen.config.template files: set CASE_SENSE_NAMES = YES for deterministic name matching across macOS and Linux.
• snapshot.py: create_enum() returns an existing enum scope instead of raising when the enum is already registered.
• builders.py: create_enum_scope() to skip enums that already exist; get_typedef_member() to normalize Doxygen’s typedef ... = ... form to using so the output format is unified.

Changelog:

[INTERNAL] [FIXED] - Fix C++ API snapshot generation crash on duplicate codegen enums
[INTERNAL] [FIXED] - Fix platform-dependent inconsistent CXX API generator output

Test Plan:

• yarn cxx-api-build completes on macOS (tested locally)
• yarn cxx-api-validate passes (tested locally on MacOS & on the Linux CI)
• validate-cxx-api-snapshots passes on Linux (tested on CI)

[cipolleschi]

Thanks for fixing the parser. I left a comment on one of the changes.

[cipolleschi]

I'm not a fan of this ad hoc handling.. Is this actually happening for only these two cases? OR can we generalize the behavior somehow.

[artus9033]

I only reproduced an edge case for these 2 cases. I've tried finding a broader approach pattern and it unfortunately modifies existing contents to use using where there used to be typedef. At least, I couldn't find a broader approach / other cases breaking the parser.