Fix OpenAI chat message selection - #1183

Summary

• use the latest chat message when dispatching OpenAI-compatible requests to agents, embeddings search, non-LLM pipelines, and workflows
• keep the default LLM path unchanged so it still receives the full conversation
• add a regression test with a system message followed by the current user message

Fixes #1182

Validation

• black --check src/python/txtai/api/routers/openai.py test/python/testapi/testopenai.py
• pylint --rcfile=.pylintrc src/python/txtai/api/routers/openai.py test/python/testapi/testopenai.py (10.00/10)
• python -m py_compile src/python/txtai/api/routers/openai.py test/python/testapi/testopenai.py
• isolated dispatch matrix covering agent, embeddings, pipeline, workflow, and unchanged full-message LLM behavior