fix(build): resolve type ambiguity and macro redefinition in unity builds

[junrushao]

Summary

• src/ffi/object.cc: Replace using namespace tvm::ffi; with a namespace alias (namespace ffi = ::tvm::ffi;) and fully qualify all type names in the static init block. This prevents ambiguous type references (e.g. Object matching both tvm::ffi::Object and tvm::runtime::Object) in downstream wasm projects that include tvm-ffi sources in a unity build.
• src/ffi/testing/testing.cc: Remove the #define TVM_FFI_DLL_EXPORT_INCLUDE_METADATA 1 from the source file to avoid macro redefinition warnings in unity builds where the macro is already set by the build system.
• CMakeLists.txt: Add target_compile_definitions(tvm_ffi_testing PRIVATE TVM_FFI_DLL_EXPORT_INCLUDE_METADATA=1) so the define is provided via the build system instead of hardcoded in source.

Test plan

• Verify existing C++ tests pass (ctest in CI)
• Verify downstream wasm unity builds compile without ambiguity errors or macro redefinition warnings

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses critical build issues related to C++ unity builds, specifically targeting type ambiguity and macro redefinitions. The changes ensure cleaner compilation for projects, especially those integrating tvm-ffi sources into WebAssembly environments, by standardizing namespace usage and centralizing macro definitions within the build system.

Highlights

• Type Ambiguity Resolution: Replaced a global using namespace tvm::ffi; with a namespace alias and fully qualified type names within a static initialization block in src/ffi/object.cc. This change specifically addresses ambiguous type references that can occur in unity builds, particularly in downstream WebAssembly projects.
• Macro Redefinition Fix: Removed a hardcoded #define TVM_FFI_DLL_EXPORT_INCLUDE_METADATA 1 from src/ffi/testing/testing.cc. This prevents macro redefinition warnings when the macro is already defined by the build system in unity build scenarios.
• CMake Build System Update: Added target_compile_definitions(tvm_ffi_testing PRIVATE TVM_FFI_DLL_EXPORT_INCLUDE_METADATA=1) to CMakeLists.txt. This ensures the TVM_FFI_DLL_EXPORT_INCLUDE_METADATA macro is provided via the build system, aligning with the removal of its hardcoded definition in the source file.

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[gemini-code-assist]

Code Review

This pull request refactors the FFI object registration by replacing a using namespace directive with explicit namespace aliases for tvm::ffi and tvm::ffi::reflection in src/ffi/object.cc. It also centralizes the definition of the TVM_FFI_DLL_EXPORT_INCLUDE_METADATA macro by moving it from src/ffi/testing/testing.cc to CMakeLists.txt, where it is now set as a compile definition for the tvm_ffi_testing target. There is no feedback to provide as no review comments were given.