test: add DataProvider props contract test

Verifies that the DataProvider in packages/ui/src/context/data.tsx still
accepts the onOpenFile prop and exports OpenFileFn. These are kilocode
additions that the extension webview depends on.

Ref: testing-gaps-and-visual-testing-plan.md §1.4

packages/kilo-vscode/tests/unit/data-provider-contract.test.ts

@@ -0,0 +1,29 @@
+/**
+ * DataProvider props contract test.
+ *
+ * The kilo-vscode webview passes an onOpenFile callback to the DataProvider
+ * from @kilocode/kilo-ui. This test verifies that the DataProvider still
+ * accepts the onOpenFile prop and that the OpenFileFn type is still defined.
+ *
+ * If upstream removes the kilocode_change additions in data.tsx, this will fail.
+ */
+
+import { describe, it, expect } from "bun:test"
+import fs from "node:fs"
+import path from "node:path"
+
+const MONOREPO_ROOT = path.resolve(import.meta.dir, "../../../..")
+const DATA_CONTEXT_FILE = path.join(MONOREPO_ROOT, "packages/ui/src/context/data.tsx")
+
+describe("DataProvider props contract", () => {
+  it("DataProvider still accepts onOpenFile prop", () => {
+    const src = fs.readFileSync(DATA_CONTEXT_FILE, "utf-8")
+    expect(src).toContain("onOpenFile")
+    expect(src).toContain("OpenFileFn")
+  })
+
+  it("openFile is exposed on the context return value", () => {
+    const src = fs.readFileSync(DATA_CONTEXT_FILE, "utf-8")
+    expect(src).toContain("openFile:")
+  })
+})