fix(ryzenai): accept OpenAI content-part arrays in chat messages

[anditherobot]

Summary

Fixes #629.

Merges text-part arrays in messages[].content into plain strings for RyzenAI and rejects unsupported media parts with HTTP 400 before streaming starts.

Scope

• Merge text parts and check for invalid media types (src/cpp/include/lemon/backends/ryzenai/ryzenai_request.h & src/cpp/server/backends/ryzenai/ryzenai_request.cpp).
• Apply the string conversion to streaming and non-streaming requests (src/cpp/server/backends/ryzenai/ryzenai_server.cpp).
• Check for invalid parts before opening the stream so errors return HTTP 400 (src/cpp/server/server.cpp).
• Test text merging, empty arrays, null values, and error cases (test/cpp/test_ryzenai_request.cpp).

Testing

• Ran ctest -R RyzenAIRequestNormalizationTest (all 16 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.