chore: remove unused codegraph exports (#5400)

Integrated into release/v3.8.42

tests/unit/copilot-codegraph-knowledge.test.ts

@@ -0,0 +1,34 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import {
+  findCallers,
+  getCodeGraphStats,
+  isCodeGraphAvailable,
+  searchSymbols,
+} from "../../src/lib/copilot/codegraphKnowledge.ts";
+
+test("CodeGraph knowledge helpers fail closed when the index is unavailable", () => {
+  assert.equal(isCodeGraphAvailable(), false);
+
+  assert.deepEqual(searchSymbols("handleChatCore"), {
+    success: false,
+    data: null,
+    error: "CodeGraph DB not found",
+    engine: "none",
+  });
+
+  assert.deepEqual(findCallers("handleChatCore"), {
+    success: false,
+    data: null,
+    error: "CodeGraph DB not found",
+    engine: "none",
+  });
+
+  assert.deepEqual(getCodeGraphStats(), {
+    success: false,
+    data: null,
+    error: "CodeGraph DB not found",
+    engine: "none",
+  });
+});