Strengthen parser required-field and FlexBuffers JSON null test oracles

[parasol-aser]

Summary

• Add four TestError() cases in ErrorTest() pinning parser diagnostics for required annotations: required on scalar table fields, required on struct fields, and missing required scalar/table fields during JSON parse.
• Strengthen ParseFlexbuffersFromJsonWithNullTest() so each int / double / null case asserts Parser::ParseFlexBuffer(...) == true and flexbuffers::VerifyBuffer(...) == true before reading values. Previously the return value of ParseFlexBuffer was silently dropped, so a parser regression would only surface as UB in the downstream AsInt64() / AsDouble() / IsNull() reads.

Coverage

Targeted lcov before/after on flattests shows src/idl_parser.cpp gaining 4 newly-hit lines (1155, 1164, 1627, 1628) — the Error() call sites for the new diagnostics.

Validation

cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release \
  -DFLATBUFFERS_BUILD_TESTS=ON -DFLATBUFFERS_BUILD_CPP17=OFF \
  -DFLATBUFFERS_BUILD_GRPCTEST=OFF -DFLATBUFFERS_BUILD_BENCHMARKS=OFF \
  -DFLATBUFFERS_INSTALL=OFF
cmake --build build --target flattests
ctest --test-dir build -R '^flattests$' --output-on-failure

Passes. Diff is test-only; no production changes.

Risk

Low. The parser-error assertions match diagnostic substrings, so they would need an update if those messages are ever reworded — consistent with the ~50 existing TestError() cases in this file.

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