fix: strip empty Read pages argument in OpenAI-to-Claude translator (#1354)

Some OpenAI-compatible providers emit optional string tool parameters
as empty strings (e.g. pages: "") instead of omitting them. Claude
Code rejects pages: "" as invalid, breaking the Read tool for
non-PDF files routed through 9router.

Add sanitizeToolArguments() that parses tool-call arguments and
removes known optional empty-string fields before emitting
input_json_delta back to Claude format. Currently handles the
Read tool pages field specifically.

Includes regression test.

Fixes #1278

Co-authored-by: JoJo <noreply@github.com>

tests/unit/openai-to-claude.test.js

@@ -8,6 +8,7 @@
 
 import { describe, it, expect } from "vitest";
 import { openaiToClaudeRequest } from "../../open-sse/translator/request/openai-to-claude.js";
+import { openaiToClaudeResponse } from "../../open-sse/translator/response/openai-to-claude.js";
 
 describe("openaiToClaudeRequest", () => {
   describe("response_format handling", () => {
@@ -121,4 +122,41 @@ describe("openaiToClaudeRequest", () => {
       expect(systemText).toContain("You must respond with valid JSON");
     });
   });
-});
+});
+
+describe("openaiToClaudeResponse", () => {
+  it("omits empty Read pages tool argument before emitting Claude input deltas", () => {
+    const state = { toolCalls: new Map() };
+    const chunk = {
+      id: "chatcmpl-test",
+      model: "gpt-test",
+      choices: [{
+        delta: {
+          tool_calls: [{
+            index: 0,
+            id: "call_read",
+            function: {
+              name: "Read",
+              arguments: JSON.stringify({
+                file_path: "/tmp/example.txt",
+                offset: 0,
+                limit: 120,
+                pages: ""
+              })
+            }
+          }]
+        }
+      }]
+    };
+
+    const result = openaiToClaudeResponse(chunk, state);
+    const inputDelta = result.find(event => event.delta?.type === "input_json_delta");
+
+    expect(inputDelta).toBeDefined();
+    expect(JSON.parse(inputDelta.delta.partial_json)).toEqual({
+      file_path: "/tmp/example.txt",
+      offset: 0,
+      limit: 120
+    });
+  });
+});