fix(anthropic): remove non-standard 'data: [DONE]' from Anthropic streaming

[cdpath]

Purpose

Fix protocol leakage in the Anthropic streaming endpoint: remove the non-standard data: [DONE]\n\n marker from all stream termination paths.

The data: [DONE] sentinel is specific to the OpenAI streaming protocol. According to the Anthropic API specification, a stream terminates with an event: message_stop block where the data payload is valid JSON (e.g., {"type": "message_stop"}).

Appending the OpenAI-style [DONE] marker to the Anthropic protocol breaks strict API gateways/middleware (such as LiteLLM) that expect all data: payloads to be JSON objects — json.loads(" [DONE]") evaluates to ['DONE'], causing an AttributeError when calling .get("type").

Test Plan

curl -s http://localhost:8000/v1/messages \
    -H "x-api-key: dummy" \
    -H "Content-Type: application/json" \
    -H "anthropic-version: 2023-06-01" \
    -d '{
      "model": "<model>",
      "messages": [{"role": "user", "content": "Say hi"}],
      "max_tokens": 20,
      "stream": true
    }'

Verify the stream ends with event: message_stop and no trailing data: [DONE].

Test Result

Before: stream ends with event: message_stop followed by data: [DONE]
After: stream ends cleanly with event: message_stop only

Standard Anthropic clients (Claude Code, official SDKs) continue to function correctly, as their state machines close upon receiving message_stop.

[gemini-code-assist]

Code Review

This pull request correctly addresses a protocol compliance issue in the Anthropic streaming endpoint by removing the non-standard data: [DONE] marker. The change is applied to all stream termination paths, including normal completion and error scenarios, ensuring the endpoint adheres to the Anthropic API specification. This fix is well-targeted and resolves an issue for downstream consumers that expect strict protocol compliance. The changes are correct and improve the API implementation.