Refuse a size whose computed side exceeds the maximum (closes #241)

A side given as 0 is computed from the source's proportions, and
nothing held it to 8192: 0x8192 of a 3000x600 source was resized to
40960x8192 and encoded. Process now refuses such a size with
ErrOutputTooLarge once it knows the source's dimensions, before the
resize and the encode, and both image routes answer it 400 with the
JSON error body. It is a processing error, not a failed fetch, so it
is never negative-cached. imgcache.MaxDimension is now
imageprocessor.MaxOutputDimension, so the request limit and the output
limit are one value. README.md says it under Routes and Encrypted URLs.

Model: opus-5-5

internal/imageprocessor/computed_side_internal_test.go

@@ -0,0 +1,52 @@
+package imageprocessor
+
+import (
+	"bytes"
+	"errors"
+	"testing"
+)
+
+// TestImageProcessor_RefusesComputedSideAboveMaximum verifies that a size
+// with one side 0, which Process computes from the input's proportions, is
+// served when that side comes out at MaxOutputDimension and refused with
+// ErrOutputTooLarge when it would be above. A 4096x1 or 1x4096 input given
+// 2 for its short side has 8192 for its long side.
+func TestImageProcessor_RefusesComputedSideAboveMaximum(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name                    string
+		inputWidth, inputHeight int
+		size                    Size
+		wantErr                 error
+	}{
+		{"width at the maximum", 4096, 1, Size{Height: 2}, nil},
+		{"width above the maximum", 4096, 1, Size{Height: 3}, ErrOutputTooLarge},
+		{"height at the maximum", 1, 4096, Size{Width: 2}, nil},
+		{"height above the maximum", 1, 4096, Size{Width: 3}, ErrOutputTooLarge},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			input := createTestJPEG(t, tt.inputWidth, tt.inputHeight)
+
+			result, err := New(Params{}).Process(t.Context(),
+				bytes.NewReader(input),
+				&Request{Size: tt.size, Format: FormatJPEG, FitMode: FitCover})
+			if result != nil {
+				t.Logf("Process() made %dx%d", result.Width, result.Height)
+
+				if max(result.Width, result.Height) != MaxOutputDimension {
+					t.Errorf("Process() made %dx%d, want a side of %d",
+						result.Width, result.Height, MaxOutputDimension)
+				}
+			}
+
+			if !errors.Is(err, tt.wantErr) {
+				t.Errorf("Process() error = %v, want %v", err, tt.wantErr)
+			}
+		})
+	}
+}