refactor(open-sse): add reasoningDelta helper, dedup thinking deltas (B4)

Centralize the reasoning_content delta shape (optional assistant role) used by
claude/gemini/kiro/codex/commandcode response translators. Keeps the cross-format
convention consistent for future translators. Output byte-for-byte identical;
golden + gate clean. Ollama left as-is (mutates existing delta object).

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

open-sse/translator/helpers/reasoningHelper.js

@@ -0,0 +1,6 @@
+// Build OpenAI delta carrying reasoning_content (optional leading assistant role)
+export function reasoningDelta(text, withRole = false) {
+  return withRole
+    ? { role: "assistant", reasoning_content: text }
+    : { reasoning_content: text };
+}