refactor(translator): P4 concern #2 encodeDataUri — dedupe base64 data-uri building

- imageHelper.encodeDataUri(mime, base64) replaces 5 inline `data:${m};base64,${d}` templates
- Applied to gemini/claude/antigravity request + gemini response translators
- Golden tests pass, identical output

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/translator/helpers/imageHelper.js

@@ -1,3 +1,8 @@
+// Build a base64 data URI from mime + base64 payload
+export function encodeDataUri(mimeType, base64) {
+  return `data:${mimeType};base64,${base64}`;
+}
+
 /**
  * Fetch a remote image URL and return it as a base64 data URI.
  * Used when upstream providers (Codex, etc.) require inline base64 images