Scope sub-third-parties per parent

Replace the many-to-many junction table with a direct
parent_third_party_id foreign key on third_parties. Each
sub-third-party now belongs to exactly one parent, making
duplicates across parents independent entities.

Replace the firstLevel boolean with an integer level field
(1 = direct, 2+ = parent level + 1) to support arbitrary
nesting depth.

Remove the createThirdPartyThirdPartyMapping and
deleteThirdPartyThirdPartyMapping mutations, the CLI
link/unlink commands, and the corresponding MCP tools.
Creating a child third party now just requires passing
parentThirdPartyId on the existing createThirdParty mutation.

The frontend walks the parentThirdParty chain to build
display names like "Name (Ancestor1/Ancestor2)" and shows
clickable ancestor links on the detail page.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

pkg/server/api/console/v1/organization_resolvers.go

@@ -1291,15 +1291,15 @@ func (r *organizationResolver) ThirdParties(ctx context.Context, obj *types.Orga
 	cursor := types.NewCursor(first, after, last, before, pageOrderBy)
 
 	var (
-		firstLevel *bool
-		query      *string
+		level *int
+		query *string
 	)
 	if filter != nil {
-		firstLevel = filter.FirstLevel
+		level = filter.Level
 		query = filter.Query
 	}
 
-	thirdPartyFilter := coredata.NewThirdPartyFilter(nil, firstLevel, query)
+	thirdPartyFilter := coredata.NewThirdPartyFilter(nil, level, query)
 
 	page, err := r.probo.ThirdParties.ListForOrganizationID(ctx, scope, obj.ID, cursor, thirdPartyFilter)
 	if err != nil {