fix(proxy): forward stream_options.include_usage to OpenAI-compatible upstreams

Claude Code's status line and the session log showed 0 tokens for every
streaming response routed through the OpenAI-compatible proxy. The root
cause was that the upstream request did not include
`stream_options: { include_usage: true }`, so the provider (LiteLLM and
other OpenAI-compatible endpoints) never emitted the final SSE chunk
that carries `usage`. The proxy's stream parser was already wired to
forward that chunk into an Anthropic-shaped `message_delta` event but
had nothing to forward.

Set `stream_options.include_usage` whenever the incoming Anthropic
request is streamed. Non-streamed requests are unaffected.

Ref: https://platform.openai.com/docs/api-reference/chat-streaming
     (search 'include_usage')

tests/unit/proxy/transformers/request-transformer.test.ts

@@ -115,4 +115,36 @@ describe('ProxyRequestTransformer', () => {
     expect(result.stop).toEqual(['A']);
     expect(result.metadata).toBeUndefined();
   });
+
+  it('sets stream_options.include_usage for streaming requests so upstreams return token usage', () => {
+    const transformer = new ProxyRequestTransformer();
+    const result = transformer.transform({
+      stream: true,
+      messages: [{ role: 'user', content: 'ping' }],
+    });
+
+    expect(result.stream).toBe(true);
+    expect(result.stream_options).toEqual({ include_usage: true });
+  });
+
+  it('omits stream_options for non-streaming requests', () => {
+    const transformer = new ProxyRequestTransformer();
+    const result = transformer.transform({
+      stream: false,
+      messages: [{ role: 'user', content: 'ping' }],
+    });
+
+    expect(result.stream).toBe(false);
+    expect(result.stream_options).toBeUndefined();
+  });
+
+  it('omits stream_options when the upstream flag is absent', () => {
+    const transformer = new ProxyRequestTransformer();
+    const result = transformer.transform({
+      messages: [{ role: 'user', content: 'ping' }],
+    });
+
+    expect(result.stream).toBe(false);
+    expect(result.stream_options).toBeUndefined();
+  });
 });