Fix memory leak in GetOverridableInitializerNames()

[Copilot]

Description

ConstSessionImpl::GetOverridableInitializerNames() was leaking the allocator-owned char* buffer after copying it into std::string, unlike the sibling functions GetInputNames() and GetOutputNames() which correctly call allocator.Free(name).

Fix: Add the missing allocator.Free(name) after emplace_back:

// Before
for (size_t i = 0; i < num_initializers; ++i) {
    char* name;
    ThrowOnError(GetApi().SessionGetOverridableInitializerName(this->p_, i, allocator, &name));
    initializer_names.emplace_back(name);  // name copied, but buffer never freed
}

// After
for (size_t i = 0; i < num_initializers; ++i) {
    char* name;
    ThrowOnError(GetApi().SessionGetOverridableInitializerName(this->p_, i, allocator, &name));
    initializer_names.emplace_back(name);
    allocator.Free(name);  // matches pattern in GetInputNames() / GetOutputNames()
}

Motivation and Context

Every call to GetOverridableInitializerNames() on a model with overridable initializers leaked one heap allocation per initializer. Reproducible by calling the function repeatedly and observing monotonically increasing resident memory.