Test that a source query in an image URL reaches the signature check

Round trips through a chi router for source queries holding /, ;, ,,
=, &, ? and a percent-escape, both as GenerateSignedURL writes them and
with characters other than ? and % percent-encoded, plus the README's
example of a source URL with a query. These fail until the fix.

Model: opus-5-5

internal/signature/signature_test.go

@@ -475,6 +475,43 @@ func TestGenerateSignedURL_WithoutQueryString(t *testing.T) {
 	}
 }
 
+// TestGenerateSignedURL_ReadmeQueryExample checks the README's example of a
+// source URL with a query, https://cdn.example.com/img?url=https://origin.example.com/my%20cat.jpg:
+// its signature with the README's key, and the path GenerateSignedURL writes
+// for it, with the "?" written as %3F and the "%" as %25.
+func TestGenerateSignedURL_ReadmeQueryExample(t *testing.T) {
+	t.Parallel()
+
+	signer := signature.New("example-signing-key-for-documentation")
+
+	req := signature.Request{
+		SourceHost:  testHost,
+		SourcePath:  "/img",
+		SourceQuery: "url=https://origin.example.com/my%20cat.jpg",
+		Width:       800,
+		Height:      600,
+		Format:      testFormatWebP,
+		Quality:     85,
+		FitMode:     testFitCover,
+		Expires:     time.Unix(1704067200, 0),
+	}
+
+	const wantSig = "37aFa9GSVCZM1kRY9SzMOk2fOg_M7Wt5gKNR4xM-FYA="
+
+	gotSig := signer.Sign(&req)
+	if gotSig != wantSig {
+		t.Errorf("Sign() = %q, want %q", gotSig, wantSig)
+	}
+
+	const wantPath = "/v1/image/cdn.example.com" +
+		"/img%3Furl=https://origin.example.com/my%2520cat.jpg/800x600.webp"
+
+	gotPath, _, _ := signer.GenerateSignedURL(&req, time.Hour)
+	if gotPath != wantPath {
+		t.Errorf("GenerateSignedURL() path = %q, want %q", gotPath, wantPath)
+	}
+}
+
 func TestParseParams(t *testing.T) {
 	t.Parallel()