Addons: copy project overrides when copying addon settings

[dallasnetprofu02-design]

Description of changes

Fixes the manual addon-settings copy flow so it migrates into the target variant's addon version and can optionally include project overrides.

Technical details

• replaces the legacy raw settings copy with migrate_addon_settings
• resolves both source and target addon versions before copying
• adds with_project_overrides to the copy request so individual addon copies can include project overrides
• adds focused tests for target-version selection and the new project-overrides flag

Closes #533

[dallasnetprofu02-design]

@martastain this is ready for review when you have a chance. I kept the change focused on the legacy manual-copy path: copy into the target variant's addon version instead of the source version, and add an explicit flag to include project overrides when copying an individual addon.