Fix loaned message ownership transfer by returning loans via shared_ptr custom deleter - #3280

Description

This PR implements the proper transfer of ownership between the RMW and the rcl layer for loaned messages, resolving a safety gap where loaned memory could be overwritten by the middleware if a user stored the shared_ptr for asynchronous processing.

Previously, rclcpp::Executor::execute_subscription unconditionally returned the loaned message immediately after the callback execution. With this change, the rcl_return_loaned_message_from_subscription is now tied to the destructor of the shared_ptr provided to the user.

• Created a custom deleter inside Subscription::handle_loaned_message which captures the rcl_subscription_t and safely returns the loaned message when the reference count drops to zero.
• Added a std::mutex (loaned_message_mutex_) in SubscriptionBase to ensure thread-safe execution of rcl_return and rcl_take.
• Removed the immediate rcl_return from executor.cpp.
• Removed the obsolete RCLCPP_WARN_ONCE about shared_ptr callbacks being unsafe for loaned messages.

Fixes #3119

Is this user-facing behavior change?

Yes, advanced users relying on zero-copy transport can now safely retain a std::shared_ptr to the loaned message. The underlying RMW memory is not returned to the pool until all references to the pointer are released.

Did you use Generative AI?

Yes, I used Antigravity (Google DeepMind agentic coding assistant) to help implement the shared_ptr custom deleter pattern and the thread-safe mutex logic across subscription_base.hpp/cpp, subscription.hpp, and executor.cpp.

Additional Information

This fix is based on the feedback from @jmachowinski to properly transfer ownership back and forth instead of introducing a new is_data_valid API. It also addresses the TODO(clalancette) in subscription_base.cpp about handling this with a custom deleter and proper locking.