fix(api): align OpenAI validation and stop handling

[sufubao]

Summary

• convert FastAPI request-validation failures into an OpenAI-style error envelope
• omit configured stop sequences from Chat Completions and Completions output
• buffer partial stop matches so streaming responses do not leak stop strings across chunk boundaries
• add regression coverage for validation failures and streaming/non-streaming stop handling

Tests

• python -m pytest -q unit_tests/server/test_openai_stop_sequences.py unit_tests/server/test_openai_validation_error.py unit_tests/server/test_api_responses.py unit_tests/server/test_qwen3_coder_stream_fc.py test/test_api/test_anthropic_extra_body.py (69 passed)
• Black check (line length 120)
• Flake8 check (repository configuration)
• git diff --check