refactor(open-sse): extract safeParseJSON util, dedup tryParseJSON (B5)

Consolidate two tryParseJSON variants into helpers/jsonUtil.js with explicit
fallback param. Preserve exact per-call semantics: openai-to-claude passthrough
(fallback=str), geminiHelper null. geminiHelper keeps tryParseJSON re-export.
No behavior change; gate: no regression.

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

open-sse/translator/helpers/geminiHelper.js

@@ -1,5 +1,7 @@
 // Gemini helper functions for translator
 
+import { safeParseJSON } from "./jsonUtil.js";
+
 // Unsupported JSON Schema constraints that should be removed for Antigravity
 export const UNSUPPORTED_SCHEMA_CONSTRAINTS = [
   // Basic constraints (not supported by Gemini API)
@@ -90,14 +92,9 @@ export function extractTextContent(content) {
   return "";
 }
 
-// Try parse JSON safely
+// Try parse JSON safely (null fallback on parse error; re-export keeps legacy API)
 export function tryParseJSON(str) {
-  if (typeof str !== "string") return str;
-  try {
-    return JSON.parse(str);
-  } catch {
-    return null;
-  }
+  return safeParseJSON(str, null);
 }
 
 // Generate request ID

open-sse/translator/helpers/jsonUtil.js

@@ -0,0 +1,5 @@
+// Safe JSON.parse: non-string passthrough; on parse error return caller-chosen `fallback`.
+export function safeParseJSON(str, fallback) {
+  if (typeof str !== "string") return str;
+  try { return JSON.parse(str); } catch { return fallback; }
+}

open-sse/translator/request/openai-to-claude.js

@@ -2,6 +2,7 @@ import { register } from "../index.js";
 import { FORMATS } from "../formats.js";
 import { CLAUDE_SYSTEM_PROMPT } from "../../config/appConstants.js";
 import { adjustMaxTokens } from "../helpers/maxTokensHelper.js";
+import { safeParseJSON } from "../helpers/jsonUtil.js";
 
 // Empty prefix matches real Claude Code behavior (no tool name prefix).
 // Previously "proxy_" was used but this is a detectable fingerprint difference.
@@ -281,7 +282,7 @@ function getContentBlocksFromMessage(msg, toolNameMap = new Map()) {
             type: "tool_use",
             id: tc.id,
             name: toolName,
-            input: tryParseJSON(tc.function.arguments)
+            input: safeParseJSON(tc.function.arguments, tc.function.arguments)
           });
         }
       }
@@ -332,16 +333,6 @@ function extractTextContent(content) {
   return "";
 }
 
-// Try parse JSON
-function tryParseJSON(str) {
-  if (typeof str !== "string") return str;
-  try {
-    return JSON.parse(str);
-  } catch {
-    return str;
-  }
-}
-
 // OpenAI -> Claude format for Antigravity (without system prompt modifications)
 function openaiToClaudeRequestForAntigravity(model, body, stream) {
   const result = openaiToClaudeRequest(model, body, stream);