Remove the code nothing in production calls (closes #312)

Service.Warm, the ImageCache interface, both GenerateSignedURL methods,
signature.ParseParams, imagerequest.ParseImagePath, four symbols of
internal/magic, the image processor's lists of supported formats,
IsEmpty and Count of the allowlist, ssrfSafeDialer, CacheStale,
ErrCacheMiss, ErrNegativeCache, ImageResponse.LastModified and the Load
methods of ContentStorage and VariantStorage had no caller outside
tests. They are gone, with the tests that tested only them.

The handlers tests sign their image URLs with generateSignedURL, a test
helper that builds the URL as the removed generator did. The SSRF dialer
tests call dialSSRFSafe with no operator blocklist, which is all
ssrfSafeDialer did. The tests of ParseImagePath and ContentStorage.Load
call ParseImageURL and LoadWithSize instead. MIMEToImageFormat,
ContentStorage.Store and CleanExpired stay, as pixa calls them.

Model: opus-5-5

internal/httpfetcher/fetch_internal_test.go

@@ -524,7 +524,7 @@ func TestFetchSemaphoreReleasedOnPartialReadClose(t *testing.T) {
 	})
 }
 
-// The dial-time re-resolution in ssrfSafeDialer is what closes the DNS
+// The dial-time re-resolution in dialSSRFSafe is what closes the DNS
 // rebinding window: even if validateURL saw a public answer earlier, the
 // dialer independently re-checks the address it is about to connect to. A full
 // rebinding simulation (a resolver returning public, then private) would mean
@@ -544,9 +544,9 @@ func TestSSRFSafeDialerBlocksPrivateTarget(t *testing.T) {
 		t.Run(addr, func(t *testing.T) {
 			t.Parallel()
 
-			_, err := ssrfSafeDialer(context.Background(), "tcp", addr)
+			_, err := dialSSRFSafe(context.Background(), "tcp", addr, nil)
 			if !errors.Is(err, ErrSSRFBlocked) {
-				t.Errorf("ssrfSafeDialer(%q) = %v, want ErrSSRFBlocked", addr, err)
+				t.Errorf("dialSSRFSafe(%q) = %v, want ErrSSRFBlocked", addr, err)
 			}
 		})
 	}
@@ -560,7 +560,7 @@ func TestSSRFSafeDialerAllowsPublicTarget(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
 
-	_, err := ssrfSafeDialer(ctx, "tcp", testPublicHost+":80")
+	_, err := dialSSRFSafe(ctx, "tcp", testPublicHost+":80", nil)
 	if err == nil {
 		t.Fatal("expected a dial error for an unreachable public target")
 	}