Add GoalStatusCode enum and get_status_code() to client goal handles

[dv-picknik]

Review draft inside our fork. Upstream, the base becomes ros2/rclcpp:rolling.

Description

This implements #2482. ClientGoalHandle::get_status() and GenericClientGoalHandle::get_status() return int8_t. Callers have to know those values are action_msgs::msg::GoalStatus::STATUS_* constants. ResultCode does not fit, because it covers only the three terminal states.

This adds rclcpp_action::GoalStatusCode, an int8_t enum class in types.hpp with all seven GoalStatus values. Both goal handles get a get_status_code() that returns it.

The status comes straight from the server's status topic, and the client does not check it. If a server sends a value outside the seven, get_status_code() returns UNKNOWN, so callers never see an unnamed enum value. get_status() still returns the raw byte.

The change only adds API, and the new GenericClientGoalHandle member is non-virtual, so existing binaries keep working.

The names are open to change. I also considered GoalState, which matches rcl_action's GOAL_STATE_*, and get_goal_status().

Would you rather deprecate get_status() on rolling and have it return the enum instead of adding a second getter? I kept this additive so it can be backported to humble, which the issue asks for. Deprecating would make it rolling-only. I can do either.

Fixes ros2#2482

Is this user-facing behavior change?

No behavior change. New API only.

Did you use Generative AI?

Yes. Claude Code with Claude Opus 5.5 wrote the code, the tests and this description. I reviewed the change, then built and tested it.

Additional Information

I tested on rolling with rclcpp and rcl built from source. All rclcpp_action tests and linters pass. The package skips cppcheck on rolling. New assertions in test_client.cpp and test_generic_client.cpp check ACCEPTED, SUCCEEDED, UNKNOWN after invalidation, and CANCELED after cancel-all. A new get_status_code_out_of_range test in each file publishes status 42 for a live goal. It fails without the range check, where get_status_code() returns 0x2A.

This branch is on an organization fork, which GitHub does not let maintainers push to. If you want changes, tell me and I will make them.

[dv-picknik]

Superseded by ros2#3292.