Prevent redirect bypasses via backslash-encoded paths (#36660)

This change tightens relative URL validation to reject raw backslashes
and `%5c` (encoded backslash), since browsers and URL normalizers can
treat backslashes as path separators. That normalization can turn
seemingly relative paths into scheme-relative URLs, creating
open-redirect risk.

Visiting below URL to reproduce the problem.

http://localhost:3000/user/login?redirect_to=/a/../\example.com

http://localhost:3000/user/login?redirect_to=/a/../%5cexample.com

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/httplib/url_test.go

@@ -23,6 +23,7 @@ func TestIsRelativeURL(t *testing.T) {
 		"foo",
 		"/",
 		"/foo?k=%20#abc",
+		"/foo?k=\\",
 	}
 	for _, s := range rel {
 		assert.True(t, IsRelativeURL(s), "rel = %q", s)
@@ -32,6 +33,8 @@ func TestIsRelativeURL(t *testing.T) {
 		"\\\\",
 		"/\\",
 		"\\/",
+		"/a/../\\b",
+		"/any\\thing",
 		"mailto:a@b.com",
 		"https://test.com",
 	}