safeweb: add support for "/" and "/foo" handler distinction (#13980)

By counting "/" elements in the pattern we catch many scenarios, but not
the root-level handler. If either of the patterns is "/", compare the
pattern length to pick the right one.

Updates https://github.com/tailscale/corp/issues/8027

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

safeweb/http_test.go

@@ -527,13 +527,13 @@ func TestGetMoreSpecificPattern(t *testing.T) {
 		{
 			desc: "same prefix",
 			a:    "/foo/bar/quux",
-			b:    "/foo/bar/",
+			b:    "/foo/bar/", // path.Clean will strip the trailing slash.
 			want: apiHandler,
 		},
 		{
 			desc: "almost same prefix, but not a path component",
 			a:    "/goat/sheep/cheese",
-			b:    "/goat/sheepcheese/",
+			b:    "/goat/sheepcheese/", // path.Clean will strip the trailing slash.
 			want: apiHandler,
 		},
 		{
@@ -554,6 +554,12 @@ func TestGetMoreSpecificPattern(t *testing.T) {
 			b:    "///////",
 			want: unknownHandler,
 		},
+		{
+			desc: "root-level",
+			a:    "/latest",
+			b:    "/", // path.Clean will NOT strip the trailing slash.
+			want: apiHandler,
+		},
 	} {
 		t.Run(tt.desc, func(t *testing.T) {
 			got := checkHandlerType(tt.a, tt.b)