feat(assignees): add set, add, and remove assignees APIs (#1045)

Implemented set, add, and remove assignees APIs.
Closes https://gitea.com/gitea/tea/issues/965 and https://gitea.com/gitea/tea/issues/966Reviewed-on: https://gitea.com/gitea/tea/pulls/1045
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Minjie Fang <wingsallen@gmail.com>

modules/task/assignees.go

@@ -0,0 +1,49 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package task
+
+import (
+	stdctx "context"
+	"fmt"
+	"strings"
+
+	gitea "gitea.dev/sdk"
+)
+
+// ResolveAssigneeOpts resolves assignee names to IssueAssigneesOption. Returns nil if names is empty.
+func ResolveAssigneeOpts(names []string) *gitea.IssueAssigneesOption {
+	names = cleanAssignees(names)
+	if len(names) == 0 {
+		return nil
+	}
+
+	return &gitea.IssueAssigneesOption{Assignees: names}
+}
+
+// ApplyAssigneeChanges adds and removes assignees on an issue or pull request.
+func ApplyAssigneeChanges(requestCtx stdctx.Context, client *gitea.Client, owner, repo string, index int64, add, rm *gitea.IssueAssigneesOption) error {
+	if rm != nil {
+		_, _, err := client.Issues.DeleteIssueAssignees(requestCtx, owner, repo, index, *rm)
+		if err != nil {
+			return fmt.Errorf("could not remove assignees: %s", err)
+		}
+	}
+	if add != nil {
+		_, _, err := client.Issues.AddIssueAssignees(requestCtx, owner, repo, index, *add)
+		if err != nil {
+			return fmt.Errorf("could not add assignees: %s", err)
+		}
+	}
+	return nil
+}
+
+func cleanAssignees(list []string) []string {
+	out := make([]string, 0, len(list))
+	for _, a := range list {
+		if strings.TrimSpace(a) != "" {
+			out = append(out, a)
+		}
+	}
+	return out
+}