fix: improve type safety for max_completion_tokens params

Remove Record<string, unknown> type annotation to let TypeScript infer
the params object type, preserving type checking on all properties.
Cast to ChatCompletionCreateParamsNonStreaming at the create() call site
to accommodate the SDK's missing max_completion_tokens type. Add unit
test for reasoning model detection regex.

Signed-off-by: majiayu000 <1835304752@qq.com>

test/unit/openAi.test.ts

@@ -0,0 +1,26 @@
+// Test the reasoning model detection regex used in OpenAiEngine.
+// Integration test with the engine is not possible because mistral.ts
+// uses require() which is unavailable in the ESM test environment.
+const REASONING_MODEL_RE = /^(o[1-9]|gpt-5)/;
+
+describe('OpenAiEngine reasoning model detection', () => {
+  it.each([
+    ['o1', true],
+    ['o1-preview', true],
+    ['o1-mini', true],
+    ['o3', true],
+    ['o3-mini', true],
+    ['o4-mini', true],
+    ['gpt-5', true],
+    ['gpt-5-nano', true],
+    ['gpt-4o', false],
+    ['gpt-4o-mini', false],
+    ['gpt-4', false],
+    ['gpt-3.5-turbo', false]
+  ])(
+    'model "%s" isReasoning=%s',
+    (model, expected) => {
+      expect(REASONING_MODEL_RE.test(model)).toBe(expected);
+    }
+  );
+});