Add get_node_names and get_subscriptions_info_by_topic

[kristophM]

Adds two safe wrappers on r2r::Node to complete the graph-introspection surface alongside the existing get_topic_names_and_types and get_publishers_info_by_topic:

• Node::get_subscriptions_info_by_topic(topic, no_mangle) — wraps rcl_get_subscriptions_info_by_topic. Returns Vec<TopicEndpointInfo> for the subscriber side, structurally identical to the existing publisher-side method.
• Node::get_node_names() — wraps rcl_get_node_names. Returns fully-qualified node names (<namespace>/<name>, with / for the root namespace).

Both follow the structural pattern set by get_publishers_info_by_topic (#80): zero-init the rcl/rmw struct, call the C function, convert, fini the array regardless of success.

Why

Without these two wrappers, r2r downstream code that does graph discovery (e.g. introspection agents, topic-graph dashboards) can see publishers per topic but cannot see subscribers, and cannot enumerate which nodes are running. We have hit this in our own ROS2 telemetry agent (RobotOpsInc/robot_agent) — the missing wrappers were the only gap stopping us from delivering a complete node-graph snapshot, and there is no obvious workaround because node_handle is pub(crate).

This is shipped under a temporary fork pin on our side; we'd much rather depend on the upstream crate.

Test plan

• Builds against ROS2 Jazzy via the same r2r build path as existing graph methods.
• Smoke-tested downstream: a freshly-created r2r::Node shows up in get_node_names() output; topic-graph snapshots now carry subscriber endpoint info matching ros2 topic info -v <topic> (verified at downstream consumer side under RobotOpsInc/robot_agent#81).

Happy to add r2r-internal tests or adjust the style/naming if helpful.