feat(branches): add rename subcommand (#939)

Implements the 'branches rename' command to rename a branch in a repository.
This wraps the Gitea API endpoint PATCH /repos/{owner}/{repo}/branches/{branch}.

Usage: tea branches rename <old_branch_name> <new_branch_name>

Example: tea branches rename -r owner/repo main factory

This resolves issue #938.

Reviewed-on: https://gitea.com/gitea/tea/pulls/939
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Matěj Cepl <mcepl@cepl.eu>
Co-committed-by: Matěj Cepl <mcepl@cepl.eu>

cmd/branches/rename_test.go

@@ -0,0 +1,46 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package branches
+
+import (
+	"testing"
+)
+
+func TestBranchesRenameArgs(t *testing.T) {
+	tests := []struct {
+		name    string
+		args    []string
+		wantErr bool
+	}{
+		{
+			name:    "valid args",
+			args:    []string{"main", "develop"},
+			wantErr: false,
+		},
+		{
+			name:    "missing both args",
+			args:    []string{},
+			wantErr: true,
+		},
+		{
+			name:    "missing new branch name",
+			args:    []string{"main"},
+			wantErr: true,
+		},
+		{
+			name:    "too many args",
+			args:    []string{"main", "develop", "extra"},
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := ValidateRenameArgs(tt.args)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("ValidateRenameArgs() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}