Remove AsBuffer accessors and regenerate test code

[agerveshi-unity]

This PR removes the custom AsBuffer accessors from the generated Swift code, along with the getBufferPointer method in Table.swift that they call.

The checked-in test code has also been regenerated. It carries two sets of changes beyond the AsBuffer removal:

1. The Swift baselines (monster_test_generated.swift) pick up bit_flags enums becoming OptionSet structs. That codegen change landed in Update to v25.12.19 #9, but the baselines were never regenerated, so this catches them up.

2. tests/union_vector/union_vector_generated.h loses 27 lines of GetFullyQualifiedName methods. These are not something this fork generates: the recipe for that file in scripts/generate_code.py does not pass --gen-name-strings, and upstream removed the same 27 lines in [TS/JS] Move TS tests to dedicated folder and deps upgrade google/flatbuffers#7508. Our copy kept them through a merge, and regenerating drops them.

Added a Yamato job that runs scripts/check_generate_code.py and scripts/check-grpc-generated-code.py alongside the flatc artifact builds, wired into Build All, so stale generated code fails the build instead of going unnoticed.