refactor: retry file remove/rename when a file is busy and clean up os detection (#38588)

Rename functions "util.Remove" (remove.go) to "util.RemoveWithRetry"
(file_retry.go) and add comments to clarify their behaviors, also add
tests.

Refactor callers: when no concurrent access (cmd cli, migration, app
init, test), use "os.Xxx" directly.

More details are in `modules/util/file_retry.go`

By the way, clean up OS (windows) detection, make FileURLToPath test
always run

modules/util/file_retry_test.go

@@ -0,0 +1,44 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package util
+
+import (
+	"io"
+	"os"
+	"syscall"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRetryWhenFileBusy(t *testing.T) {
+	var callCount int
+	testErrPath := &os.PathError{Op: "test", Path: "test", Err: syscall.EBUSY}
+	fn := func() error {
+		if callCount == 2 {
+			return nil
+		}
+		callCount++
+		return testErrPath
+	}
+
+	callCount = 0
+	err := retryWhenFileBusyInternal(1, time.Millisecond, fn)
+	assert.Equal(t, testErrPath, err)
+	assert.Equal(t, 1, callCount)
+
+	callCount = 0
+	err = retryWhenFileBusyInternal(10, time.Millisecond, fn)
+	assert.NoError(t, err)
+	assert.Equal(t, 2, callCount)
+
+	callCount = 0
+	err = retryWhenFileBusyInternal(10, time.Millisecond, func() error {
+		callCount++
+		return io.ErrUnexpectedEOF
+	})
+	assert.Equal(t, io.ErrUnexpectedEOF, err)
+	assert.Equal(t, 1, callCount)
+}