feat: enhance response serialization for compact mode

- Added support for preserving native response.compaction payloads when compact serialization is requested in `serializeResponsesFinalPayload`.
- Implemented serialization of compact mode as `response.compaction` instead of an ordinary response object.
- Introduced a new type `ResponsesFinalSerializationMode` to manage serialization modes.
- Refactored `serializeResponsesFinalPayload` to handle different serialization modes and usage payloads.
- Added tests to ensure correct behavior for compact serialization and preservation of upstream payloads.
- Updated favicon assets and documentation configuration for VitePress.

src/server/routes/proxy/architecture-boundaries.test.ts

@@ -114,6 +114,7 @@ describe('proxy route architecture boundaries', () => {
     expect(source).not.toContain('const consumeSseBuffer = (incoming: string): string => {');
     expect(source).not.toContain('openAiResponsesTransformer.aggregator.complete(');
     expect(source).not.toContain('reply.raw.end();');
+    expect(source).toContain('reply.hijack();');
     expect(source).toContain('openAiResponsesTransformer.proxyStream.createSession(');
   });
 });

src/server/routes/proxy/responses.compact-upstream.test.ts

@@ -137,4 +137,49 @@ describe('responses proxy compact upstream routing', () => {
     const [targetUrl] = fetchMock.mock.calls[0] as [string, any];
     expect(targetUrl).toContain('/v1/responses/compact');
   });
+
+  it('preserves native response.compaction payloads instead of coercing them into object=response', async () => {
+    fetchMock.mockResolvedValue(new Response(JSON.stringify({
+      id: 'cmp_123',
+      object: 'response.compaction',
+      input_tokens: 1234,
+      output_tokens: 321,
+      total_tokens: 1555,
+      output: [
+        {
+          id: 'rs_123',
+          type: 'compaction',
+          encrypted_content: 'enc-compact-payload',
+        },
+      ],
+    }), {
+      status: 200,
+      headers: { 'content-type': 'application/json' },
+    }));
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/v1/responses/compact',
+      payload: {
+        model: 'gpt-5.2',
+        input: 'hello',
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+    expect(response.json()).toEqual({
+      id: 'cmp_123',
+      object: 'response.compaction',
+      input_tokens: 1234,
+      output_tokens: 321,
+      total_tokens: 1555,
+      output: [
+        {
+          id: 'rs_123',
+          type: 'compaction',
+          encrypted_content: 'enc-compact-payload',
+        },
+      ],
+    });
+  });
 });

src/server/routes/proxy/responses.compact.test.ts

@@ -29,4 +29,24 @@ describe('responses proxy compact route', () => {
       },
     });
   });
+
+  it('rejects streaming /v1/responses/compact requests because compact is non-streaming', async () => {
+    const response = await app.inject({
+      method: 'POST',
+      url: '/v1/responses/compact',
+      payload: {
+        model: 'gpt-5.2',
+        input: 'hello',
+        stream: true,
+      },
+    });
+
+    expect(response.statusCode).toBe(400);
+    expect(response.json()).toEqual({
+      error: {
+        message: 'stream is not supported on /v1/responses/compact',
+        type: 'invalid_request_error',
+      },
+    });
+  });
 });