fix(image-tools): surface Sharp encode failures instead of "Error: Error" (#534)

Wrap convert and gif-tools process functions so a Sharp .toBuffer() failure carries an authored SafeError title (and the original as cause) rather than a scrubbed "Error: Error".

apps/api/src/lib/image-error.ts

@@ -0,0 +1,40 @@
+import { isSafeMessageError, isToolInputError, SafeError } from "@snapotter/shared";
+import type { ToolProcessCtx } from "../routes/tool-factory.js";
+
+type ImageProcess<T> = (
+  inputBuffer: Buffer,
+  settings: T,
+  filename: string,
+  ctx?: ToolProcessCtx,
+) => Promise<{ buffer: Buffer; filename: string; contentType: string }>;
+
+/**
+ * Wrap an image tool's process function so an otherwise-opaque failure (most
+ * often a Sharp `.toBuffer()` that throws an empty-message Error) surfaces a
+ * safe, authored title instead of "Error: Error".
+ *
+ * The API's Sentry scrubber replaces any non-SafeError message with a type-only
+ * value (see `rebuildErrorValue`), so a bare Sharp failure is undiagnosable.
+ * Re-throwing as a SafeError makes the title survive while the original error is
+ * kept as `cause`, preserving its stack and exact location. Errors we already
+ * author (SafeError) or that flag bad user input (ToolInputError) pass through
+ * untouched so their class is not masked.
+ */
+export function withImageEncodeContext<T>(
+  message: string,
+  codeOf: (settings: T) => string,
+  process: ImageProcess<T>,
+): ImageProcess<T> {
+  return async (inputBuffer, settings, filename, ctx) => {
+    try {
+      return await process(inputBuffer, settings, filename, ctx);
+    } catch (err) {
+      if (isSafeMessageError(err) || isToolInputError(err)) throw err;
+      throw new SafeError(message, {
+        kind: "bug",
+        code: codeOf(settings),
+        cause: err instanceof Error ? err : new Error(String(err)),
+      });
+    }
+  };
+}