fix(openai): restore adapter→builtin→override precedence in createModel

The merge order in createModel spread builtIn.compat first then this.compat,
so the adapter's generic default (maxTokensField: 'max_tokens') silently
clobbered the model-specific override that the built-in entry sets
('max_completion_tokens' for reasoning-capable models). Result: every
reasoning model loaded via createModel sent the wrong field name and OpenAI
returned 400 "Unsupported parameter: 'max_tokens'". Same bug applied to
headers. The mock-mode unit tests didn't catch it because the mocked fetch
never validated the request body — the live smoke test caught it on the
first real call.

Swap to: adapter defaults → built-in catalog → caller overrides, so the
most specific source wins. Adds two regression tests.

Author: yyyyaaa

packages/openai/__tests__/openai.test.ts

@@ -132,6 +132,20 @@ describe('OpenAIAdapter', () => {
     expect(message.usage.totalTokens).toBe(40);
   });
 
+  it('createModel preserves model-specific compat overriding adapter defaults', () => {
+    const adapter = new OpenAIAdapter({ apiKey: 'k' });
+    const model = adapter.createModel('gpt-5.4-nano');
+    expect(model.compat?.maxTokensField).toBe('max_completion_tokens');
+  });
+
+  it('createModel lets caller overrides win over both', () => {
+    const adapter = new OpenAIAdapter({ apiKey: 'k' });
+    const model = adapter.createModel('gpt-5.4-nano', {
+      compat: { maxTokensField: 'max_tokens' },
+    });
+    expect(model.compat?.maxTokensField).toBe('max_tokens');
+  });
+
   it('falls back to built-in models when no API key is configured', async () => {
     const adapter = new OpenAIAdapter();
     const models = await adapter.listModels();

packages/openai/src/index.ts

@@ -389,8 +389,8 @@ export class OpenAIAdapter {
       return {
         ...builtIn,
         baseUrl: this.baseUrl,
-        headers: { ...(builtIn.headers ?? {}), ...(this.defaultHeaders ?? {}), ...(overrides?.headers ?? {}) },
-        compat: { ...(builtIn.compat ?? {}), ...(this.compat ?? {}), ...(overrides?.compat ?? {}) },
+        headers: { ...(this.defaultHeaders ?? {}), ...(builtIn.headers ?? {}), ...(overrides?.headers ?? {}) },
+        compat: { ...(this.compat ?? {}), ...(builtIn.compat ?? {}), ...(overrides?.compat ?? {}) },
         ...overrides,
       };
     }