fix: represent a deleted assignee team as a Ghost team (#38413)

Fixes #35472.

`Comment.LoadAssigneeUserAndTeam` already has a Ghost user fallback
for a deleted assignee user, but the parallel branch for a deleted
assignee team just swallowed the not-found error and left
`AssigneeTeam` as `nil`. This is inconsistent (the reporter's example
shows `assignee` becoming a Ghost user while `assignee_team` becomes
`null`), and it's also a latent nil pointer bug: other code that
assumes `AssigneeTeam` is set once this function returns without
error will panic.

Added `organization.NewGhostTeam()` / `Team.IsGhost()`, mirroring the
existing `user_model.NewGhostUser()` / `User.IsGhost()` pattern, and
used it in the same fallback branch.

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/organization/team.go

@@ -6,6 +6,7 @@ package organization
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"strings"
 
@@ -92,6 +93,15 @@ func (t *Team) IsPublic() bool  { return t.Visibility.IsPublic() }
 func (t *Team) IsLimited() bool { return t.Visibility.IsLimited() }
 func (t *Team) IsPrivate() bool { return t.Visibility.IsPrivate() }
 
+const (
+	ghostTeamID   = -1
+	ghostTeamName = "(deleted team)"
+)
+
+func newGhostTeam() *Team {
+	return &Team{ID: ghostTeamID, Name: ghostTeamName, LowerName: ghostTeamName}
+}
+
 // CanNonMemberReadMeta reports whether a non-member, non-owner doer may read
 // the team's metadata, based on the team's visibility tier and the parent org's
 // visibility. Privileged callers (site admins, org owners, team members) are
@@ -270,6 +280,17 @@ func GetTeamByID(ctx context.Context, teamID int64) (*Team, error) {
 	return t, nil
 }
 
+func GetPossibleTeamByID(ctx context.Context, teamID int64) (int64, *Team, error) {
+	t, err := GetTeamByID(ctx, teamID)
+	if errors.Is(err, util.ErrNotExist) {
+		t = newGhostTeam()
+		return t.ID, t, nil
+	} else if err != nil {
+		return 0, nil, err
+	}
+	return t.ID, t, nil
+}
+
 // IncrTeamRepoNum increases the number of repos for the given team by 1
 func IncrTeamRepoNum(ctx context.Context, teamID int64) error {
 	_, err := db.GetEngine(ctx).Incr("num_repos").ID(teamID).Update(new(Team))