Allow multiple projects per issue and pull requests (#36784)

Add ability to add and remove multiple projects per issue
and pull request.

Resolve #12974

---------

Signed-off-by: Icy Avocado <avocado@ovacoda.com>
Co-authored-by: Tyrone Yeh <siryeh@gmail.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: OpenCode (gpt-5.2-codex) <opencode@openai.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

services/convert/issue.go

@@ -95,6 +95,13 @@ func toIssue(ctx context.Context, doer *user_model.User, issue *issues_model.Iss
 		apiIssue.Milestone = ToAPIMilestone(issue.Milestone)
 	}
 
+	if err := issue.LoadProjects(ctx); err != nil {
+		return &api.Issue{}
+	}
+	if len(issue.Projects) > 0 {
+		apiIssue.Projects = ToAPIProjectList(issue.Projects)
+	}
+
 	if err := issue.LoadAssignees(ctx); err != nil {
 		return &api.Issue{}
 	}