Improve network connection management and sync with NetworkManager events

[dgdavid]

Problem

Until now, Agama was not aware when network connections were added or removed by third-party tools like nmcli, causing the internal network state to become out of sync with the system. Additionally, the UI lacked key functionality for managing connections, such as identifying associated devices, performing contextual actions, and creating or removing connections.

Solution

• Implemented automatic detection of NetworkManager connection add/remove events to keep the internal state synchronized with the system (see Detect NetworkManager connection add/remove events and maintain the internal state in sync #3244).
• Restored and improved the connections table to better display associated devices.
• Added contextual actions for connections (view, edit, connect, delete).
• Enabled creating and removing connections directly from the UI.
• Extended IP configuration with support for the dnsSearchList field.
• Removed unused actions (e.g., AddConnection, SetPorts) to simplify the backend.

Note for reviewers

Since there is a lot of code, slightly pre-reviewed, probably would be better to check this one by manual testing using ISO available at https://download.opensuse.org/repositories/systemsmanagement:/Agama:/branches:/network-202603/images/iso/

Testing

• Added a new unit test.
• Tested manually.

Related to

• Detect NetworkManager connection add/remove events and maintain the internal state in sync #3244
• Network backend improvements #3276
• refactor(web): enhance network page #3247
• https://trello.com/c/jvgwGFIl/ https://bugzilla.suse.com/show_bug.cgi?id=1247441
• https://jira.suse.com/browse/PED-15913

[imobachgs]

I was unable to do a deep review of the code, but I checked the approach and gave it a try. It is far better that what we had before.

Thanks!