test: /v1/image/ answers an invalid q with 400 (closes #134)

Route tests for q=banana, q=0, q=101, an empty q= and q=&q=500: each
must be a 400 whose error names q and the value. They fail on next,
where each is served at the default quality 85.

Model: opus-5-5

internal/handlers/handlers_internal_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"database/sql"
+	"encoding/json"
 	"image"
 	"image/color"
 	"image/jpeg"
@@ -291,3 +292,57 @@ func TestHandleImage_InvalidFitMode_Returns400(t *testing.T) {
 		t.Fatalf("status = %d, want %d", status, http.StatusBadRequest)
 	}
 }
+
+// TestHandleImage_InvalidQuality_Returns400 verifies that the plain image
+// route answers a q that is not a whole number from 1 to 100, an empty one
+// included, with 400 naming q and the value, instead of serving the image at
+// the default quality 85.
+func TestHandleImage_InvalidQuality_Returns400(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		query, wantError string
+	}{
+		{"q=banana", `invalid q: not a number, got "banana"`},
+		{"q=0", `invalid q: must be from 1 to 100, got "0"`},
+		{"q=101", `invalid q: must be from 1 to 100, got "101"`},
+		{"q=", `invalid q: not a number, got ""`},
+		{"q=&q=500", `invalid q: not a number, got ""`},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.query, func(t *testing.T) {
+			t.Parallel()
+
+			fix := setupTestHandler(t)
+
+			r := chi.NewRouter()
+			r.Get("/v1/image/*", fix.handler.HandleImage())
+
+			req := httptest.NewRequestWithContext(t.Context(), http.MethodGet,
+				"/v1/image/"+fix.goodHost+"/images/photo.jpg/50x50.jpeg?"+tt.query, nil)
+			rec := httptest.NewRecorder()
+
+			r.ServeHTTP(rec, req)
+
+			if rec.Code != http.StatusBadRequest {
+				t.Fatalf("status = %d, want %d", rec.Code, http.StatusBadRequest)
+			}
+
+			t.Logf("GET %s: %d %s", req.URL, rec.Code, rec.Body)
+
+			var body struct {
+				Error string `json:"error"`
+			}
+
+			err := json.NewDecoder(rec.Body).Decode(&body)
+			if err != nil {
+				t.Fatalf("decoding response body: %v", err)
+			}
+
+			if body.Error != tt.wantError {
+				t.Errorf("error = %q, want %q", body.Error, tt.wantError)
+			}
+		})
+	}
+}