- Fix get_varint64_ifdef() to return unconditional guard when any 64-bit
varint field has no ifdef (None in ifdefs set)
- Wrap USE_API_VARINT64 define with #ifndef to avoid redefinition warnings
when esphome/core/defines.h also defines it (test/IDE builds)
- Clarify proto.h comment about uint32_t shift behavior at byte 4
- Add namespace to generated api_pb2_defines.h for linter compliance