Add support for reporting target entities with no links in entity_mesh

Problem/Motivation

The entity_mesh module currently saves only those target entities that have at least one link or connection from a source entity. As a result, entities that have no relationships (i.e., targets with no links) are not included in the report or visualization. This limitation makes it difficult to identify isolated entities, which is important for understanding disconnected or orphaned nodes within the data set.

Steps to reproduce

1. Configure entity_mesh with a set of entities where some nodes do not have any incoming or outgoing links.
2. Generate a report or view the visualization.
3. Observe that entities with no links are missing from the output.

Proposed resolution

Enhance the report generation and visualization logic so that all target entities are included, even if they have no links. For entities with no links, add them to the report and assign them a new category, such as no-links, for sources with no connections. This will allow users to easily identify entities that are not related to any other nodes in the mesh, improving data completeness and supporting further analysis or cleanup.

Remaining tasks

• Update the data retrieval logic to include all entities, not just those with links.
• Modify the report and visualization to display nodes with no links, marking them with the no-links category.
• Ensure that the UI or report format clearly distinguishes these isolated entities.
• Test with different data sets to confirm correct behavior.
• Update any relevant documentation to describe the new behavior.

User interface changes

Nodes with no links will be displayed in the report, grouped or labeled under a new category called no-links. This improves the user's ability to detect and review disconnected entities.

API changes

None expected.

Data model changes

None expected.