Refuse a fit=outside size whose output exceeds the maximum (closes #257)

fit=outside scales the source to cover the requested size, and the side
scaled past it was held to no maximum: 1x8192?fit=outside of a 3000x600
source was resized to 40960x8192 and encoded. resize now refuses such an
output with ErrOutputTooLarge before resizing, so both image routes answer
it 400 with the error a size with one side 0 already gets. The other fit
modes make the requested size or one within it. README.md and TODO.md
say so.

Model: opus-5-5

internal/server/fit_outside_internal_test.go

@@ -19,11 +19,17 @@ import (
 func TestImageProxyRefusesFitOutsideAboveMaximum(t *testing.T) {
 	t.Parallel()
 
-	// The sources, by their upstream paths
+	// The upstream paths of the sources
+	const (
+		widePath   = "/wide.png"
+		rowPath    = "/row.png"
+		columnPath = "/column.png"
+	)
+
 	sources := map[string][]byte{
-		"/wide.png":   encodeTestPNG(t, 3000, 600),
-		"/row.png":    encodeTestPNG(t, 8192, 1),
-		"/column.png": encodeTestPNG(t, 1, 8192),
+		widePath:   encodeTestPNG(t, 3000, 600),
+		rowPath:    encodeTestPNG(t, 8192, 1),
+		columnPath: encodeTestPNG(t, 1, 8192),
 	}
 
 	upstream := httptest.NewServer(http.HandlerFunc(
@@ -46,10 +52,10 @@ func TestImageProxyRefusesFitOutsideAboveMaximum(t *testing.T) {
 		// wantWidth and wantHeight are the served image's, 0 for a refusal
 		wantWidth, wantHeight int
 	}{
-		{"/wide.png", 1, 8192, 0, 0},
-		{"/row.png", 1, 8192, 0, 0},
-		{"/column.png", 8192, 1, 0, 0},
-		{"/wide.png", 1, 1200, 6000, 1200},
+		{widePath, 1, 8192, 0, 0},
+		{rowPath, 1, 8192, 0, 0},
+		{columnPath, 8192, 1, 0, 0},
+		{widePath, 1, 1200, 6000, 1200},
 	}
 
 	for _, tt := range tests {