Hardening: add null guards, bounds checks, and file saver safety to language IDL generators

[anagnorisis2peripeteia]

Description

This PR strengthens defensive engineering across FlatBuffers language IDL generators (Swift, TypeScript, Rust, Python, Java, Kotlin, C#, Dart, PHP, Lobster, Text, and gRPC):

1. src/idl_gen_swift.cpp:

   • Check parser_.opts.file_saver against nullptr before invoking SaveFile.
   • Add null guards in parser_.enums_.vec and parser_.structs_.vec loops before dereferencing **it.

2. src/idl_gen_ts.cpp:

   • Check definition.defined_namespace against nullptr in SaveType.
   • Guard parser_.opts.file_saver against nullptr.
   • In TrackNsDef child namespace re-export loops, guard it.second.ns and it2.second.ns against null and verify !it2.second.ns->components.empty() before iterator subtraction (end() - count).

3. src/idl_gen_rust.cpp:

   • Check parser.opts.file_saver against nullptr in GenerateRustModuleRootFile.
   • Guard symbol.defined_namespace and parser_.opts.file_saver against null in GenerateIndividualFiles.
   • Guard parser_.opts.file_saver against null in GenerateSingleFile.

4. src/idl_gen_python.cpp:

   • Guard parser_.opts.file_saver in PythonStubGenerator::SaveFile and PythonGenerator::SaveType.
   • In PythonGenerator::SaveType, verify path_.size() <= directories.size() before calling std::string::find to prevent out-of-bounds indexing.
   • Null-check parser_.opts.file_saver when creating empty __init__.py files.

5. src/idl_gen_java.cpp:

   • Guard parser_.opts.file_saver against nullptr in JavaGenerator::SaveType.

6. src/idl_gen_kotlin.cpp & src/idl_gen_kotlin_kmp.cpp:

   • Guard parser_.opts.file_saver against nullptr in KotlinGenerator::SaveType and KotlinKmpGenerator::SaveType.

7. src/idl_gen_csharp.cpp:

   • Guard options.file_saver against nullptr in CSharpGenerator::SaveType.

8. src/idl_gen_dart.cpp:

   • Guard parser_.opts.file_saver against nullptr before invoking SaveFile.

9. src/idl_gen_php.cpp:

   • Guard def.defined_namespace and parser_.opts.file_saver against null in PhpGenerator::SaveType.

10. src/idl_gen_lobster.cpp:

    • Guard *it in structs loop against null.
    • Guard parser_.opts.file_saver against null in LobsterGenerator::generate.

11. src/idl_gen_text.cpp:

    • Guard parser.opts.file_saver against null in GenTextFile flexbuffers and normal text paths.

12. src/idl_gen_grpc.cpp:

    • Guard def and def->defined_namespace in GoGRPCGenerator and JavaGRPCGenerator.
    • Guard *it against null in parser.services_.vec iteration across GenerateGoGRPC, GenerateCppGRPC, GenerateJavaGRPC, GeneratePythonGRPC, GenerateSwiftGRPC.
    • Guard parser.opts.file_saver against null across C++, Java, Swift, and TypeScript gRPC generators.

13. tests/test.cpp:

    • Added unit test IdlLanguageGeneratorsSafetyTest() verifying that GenerateTextFile fails cleanly and returns SaveFile failed when opts.file_saver == nullptr.

All tests compile cleanly under -DFLATBUFFERS_STRICT_MODE=ON and pass with flattests and flatc_cpp_tests.py.