fix(openai): clarify GPT-5.6 Chat Completions tool error

[zyssyz123]

Summary

• recognize the OpenAI 400 returned when GPT-5.6 uses function tools with reasoning through Chat Completions
• replace that known upstream error with actionable guidance to switch API Protocol to Responses
• preserve the existing generic error mapping for unrelated bad requests and custom endpoint behavior

Testing

• uv run pytest tests/test_routing.py -q (51 passed)
• uv run pytest -q (155 passed, 63 skipped live tests)
• uv run ruff check models/common_openai.py tests/test_routing.py
• uv run ruff format --check models/common_openai.py tests/test_routing.py
• packaged with dify-plugin package and verified the archive
• deployed the package to the remote dev environment and reproduced the Agent request end to end
• verified Chat Completions without tools, Chat Completions with tools plus reasoning_effort=none, and Responses with tools

Linear: DIFY-2818