fix(tools): harden tool schemas for strict providers

src/agents/schema/typebox.ts

@@ -0,0 +1,27 @@
+import { Type } from "@sinclair/typebox";
+
+type StringEnumOptions<T extends readonly string[]> = {
+  description?: string;
+  title?: string;
+  default?: T[number];
+};
+
+// NOTE: Avoid Type.Union([Type.Literal(...)]) which compiles to anyOf.
+// Some providers reject anyOf in tool schemas; a flat string enum is safer.
+export function stringEnum<T extends readonly string[]>(
+  values: T,
+  options: StringEnumOptions<T> = {},
+) {
+  return Type.Unsafe<T[number]>({
+    type: "string",
+    enum: [...values],
+    ...options,
+  });
+}
+
+export function optionalStringEnum<T extends readonly string[]>(
+  values: T,
+  options: StringEnumOptions<T> = {},
+) {
+  return Type.Optional(stringEnum(values, options));
+}