Fix missing namespace qualifier in Pack()

[tmimmanuel]

Summary

• Fix C++ code generator to emit fully namespace-qualified Create* calls in
  Pack() when referencing tables from different namespaces
• Add test schema and round-trip Pack/UnPack test for cross-namespace table references

Problem

When a table in namespace foo references a table in namespace native, the
generated Pack() function emitted CreateTableWithNative(...) instead of
native::CreateTableWithNative(...), causing compilation errors.

Fix

Two locations in GenCreateParam in src/idl_gen_cpp.cpp now use
WrapInNameSpace() to qualify Create* calls with the referenced table's
namespace:

• Scalar table fields (e.g. c1: native.TableWithNative)
• Vector of table fields (e.g. c2: [native.TableWithNative])

Test plan

• New cross_namespace_pack_test.fbs schema with cross-namespace table references
• New CrossNamespacePackTest() in test.cpp verifies Pack/UnPack round-trip
• flattests passes locally

Closes #8948

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[jtdavis777]

this fix looks great -- it does appear that the new tests don't explicitly validate the interaction with the native API? Though looking over the PR maybe that detail isn't necessary for this bug?

[tmimmanuel]

you are right that the test doesn't use native_type. The namespace qualification fix is in the table Create* code path in GenCreateParam, which is the same path regardless of whether native_type is set on the referenced table. Adding native_type would require custom native type headers and Pack/UnPack implementations but i think we don't need it for this specific bug.