Ensure latched messages are updated on every split

Summary

This fixes an issue with #1850 - latched messages from the same publisher are not updated in the Recorder.

Steps to Reproduce

Create and run a node that periodically publishes latched messages, for example:
```
#!/usr/bin/env python3
import rospy
from std_msgs.msg import String
```

pub = rospy.Publisher('chatter', String, latch=True) rospy.init_node('talker') for i in range(20): pub.publish(str(i)) rospy.sleep(5)


2. Record the latched topic in splits:
`rosrun rosbag record -a --repeat-latched --duration 10 --split`
3. Check messages on the target topic in the last split:
`rostopic echo -b <last_split>.bag chatter`

### Expected behavior
The **first message** in the **last split** is the value of the latched topic when the split was started (e.g., `data: "18"`).

### Actual behavior
The **first message** in the **last split** is the value of the latched topic when the `rosbag record` was run (e.g., `data: "0"`).

## Implications
This behavior causes issues when recording latched topics that are not often updated. If the `rosbag record` is a long-running process, the latched topic values in some splits may be several hours old (if the publisher does not send an update within the split time frame).

## Cause
Latched messages are stored as a `std::map<std::pair<std::string, std::string>, OutgoingMessage>`, where the key is a pair of strings (topic and publisher name) and the value is the latched message.

When updating the `latched_msgs_` map, the `std::map::insert()` function is used. This behaves as expected when the key does not yet exist in the map. However, if the key exists, the old value is not updated:
> Because element keys in a map are unique, the insertion operation checks whether each inserted element has a key equivalent to the one of an element already in the container, and if so, the element is not inserted, returning an iterator to this existing element
>
> -- <cite>[std::map::insert](https://cplusplus.com/reference/map/map/insert/)</cite>

This effectively means that the first value (latched message) that was inserted into the map will persist across the whole `rosbag record`.

## Solution
Check for insertion success and assign value manually if required (see commit).

ros / ros_comm

Ensure latched messages are updated on every split #2261

Summary

Steps to Reproduce