chore(cmake): carry option-driven defines on pybind11_headers INTERFACE (#6130)

* chore(cmake): carry option-driven defines on pybind11_headers INTERFACE

PYBIND11_DISABLE_HANDLE_TYPE_NAME_DEFAULT_IMPLEMENTATION and
PYBIND11_SIMPLE_GIL_MANAGEMENT were directory-scoped
add_compile_definitions, which only reached the master-project test
build. On the pybind11_headers INTERFACE (like
PYBIND11_INTERNALS_VERSION) they also apply in add_subdirectory mode
and in the exported/installed targets.

Assisted-by: ClaudeCode:claude-fable-5

* test(cmake): verify option-driven defines reach consumer targets

Extend subdirectory_target and installed_target to check that
PYBIND11_DISABLE_HANDLE_TYPE_NAME_DEFAULT_IMPLEMENTATION and
PYBIND11_SIMPLE_GIL_MANAGEMENT propagate through the pybind11_headers
usage requirements. Fails without the parent commit.

Assisted-by: ClaudeCode:claude-fable-5