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>

models/issues/issue_test.go

@@ -424,10 +424,10 @@ func TestIssueLoadAttributes(t *testing.T) {
 		}
 		if issue.ID == int64(1) {
 			assert.Equal(t, int64(400), issue.TotalTrackedTime)
-			assert.NotNil(t, issue.Project)
-			assert.Equal(t, int64(1), issue.Project.ID)
+			assert.NotEmpty(t, issue.Projects)
+			assert.Equal(t, int64(1), issue.Projects[0].ID)
 		} else {
-			assert.Nil(t, issue.Project)
+			assert.Empty(t, issue.Projects)
 		}
 	}
 }