Fix CLI namespace expansion for expand_gz_topic_names

[AyushSuri8]

Description:
Fixes #702.

The Issue:
When creating a bridge via CLI string parsing (e.g., topic@ros_type@gz_type), the expand_gz_topic_names parameter was being completely ignored, whereas it worked correctly when passed via a YAML config file.

The Fix:
Updated the argument parsing loop in parameter_bridge.cpp to:

1. Fetch the expand_gz_topic_names parameter and the node's current namespace via get_namespace().
2. Safely prepend the ROS namespace to config.gz_topic_name when the parameter is set to true, preventing double slashes.

Testing:
Reproduced the original issue using the osrf/ros:humble-simulation Docker container. Built this branch from source and verified that running:
ros2 run ros_gz_bridge parameter_bridge chatter@std_msgs/msg/String@ignition.msgs.StringMsg --ros-args -p expand_gz_topic_names:=true -r __ns:=/demo

Now correctly results in ign topic -l outputting: /demo/chatter

[ahcorde]

Do you mind to target rolling ? and add some tests?