fix(streaming): replace null delta.role with \"assistant\" in chat completion chunks

[anditherobot]

Summary

Fixes #350.

Replaces role: null with "assistant" in chat stream chunks so client libraries do not crash during streaming.

Scope

• Rewrite null delta.role values to "assistant" in chat streams (src/cpp/server/streaming_proxy.cpp).
• Turn on role normalization for chat stream endpoints (src/cpp/server/wrapped_server.cpp).
• Test valid roles, null roles, thinking-block chunks, and raw stream passthrough (test/cpp/test_streaming_role_normalization.cpp).

Testing

• Ran ctest -R StreamingRoleNormalizationTest (all 8 tests pass).

Documentation

• No documentation changes needed.

Breaking Changes

• None.

AI-Assisted Contribution

• This contribution was created with the assistance of AI tools.
• I have reviewed the changes and take full responsibility for the code.
• I have tested the changes and verified that they work as expected.