One image request type instead of five (closes #307)

The new package internal/imagerequest holds the request type, its two
parts Source and Transform, the Format and Fit types, the dimension and
fit checks, the /v1/image/ path parsing and the variant key; it imports
no other package of pixa. imgcache.ParsedURL, imgcache.ImageRequest,
signature.Request, imageprocessor.Request, the second Size, format and
fit types and the conversions between them are gone. The image
processor takes a Transform only. encurl keeps a private struct with the
token's CBOR field names, so tokens made before still decode. VariantKey
moves with the function that makes it. Tests follow the moved and
renamed code; the tests of the removed conversions go with them.

Model: opus-5-5

internal/imagerequest/validate_internal_test.go

@@ -0,0 +1,72 @@
+package imagerequest
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestValidateImageRequest(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		req     Request
+		wantErr error
+	}{
+		{
+			name: "within bounds",
+			req: Request{Transform: Transform{
+				Size: Size{Width: 800, Height: 600}, FitMode: FitCover,
+			}},
+		},
+		{
+			name: "original size and empty fit",
+			req:  Request{Transform: Transform{Size: Size{Width: 0, Height: 0}}},
+		},
+		{
+			name: "width over limit",
+			req: Request{Transform: Transform{
+				Size: Size{Width: MaxDimension + 1, Height: 600},
+			}},
+			wantErr: ErrDimensionTooLarge,
+		},
+		{
+			name: "height over limit",
+			req: Request{Transform: Transform{
+				Size: Size{Width: 800, Height: MaxDimension + 1},
+			}},
+			wantErr: ErrDimensionTooLarge,
+		},
+		{
+			name:    "negative width",
+			req:     Request{Transform: Transform{Size: Size{Width: -1, Height: 600}}},
+			wantErr: ErrInvalidSize,
+		},
+		{
+			name: "invalid fit mode",
+			req: Request{Transform: Transform{
+				Size: Size{Width: 800, Height: 600}, FitMode: "bogus",
+			}},
+			wantErr: ErrInvalidFitMode,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			err := ValidateImageRequest(&tt.req)
+			if tt.wantErr == nil {
+				if err != nil {
+					t.Fatalf("ValidateImageRequest() error = %v, want nil", err)
+				}
+
+				return
+			}
+
+			if !errors.Is(err, tt.wantErr) {
+				t.Fatalf("ValidateImageRequest() error = %v, want %v", err, tt.wantErr)
+			}
+		})
+	}
+}