Serialize reasoning-only assistant messages with content

[willccbb]

Summary

• send empty string content when replaying reasoning-only assistant messages to OpenAI-compatible chat completions providers
• preserve reasoning_content on the replayed assistant message
• add regression coverage for the prompt normalization case
• bump verifiers to 0.1.15.dev15 so cookbook evals can consume the fix

Tests

• uv run ruff check verifiers/clients/openai_chat_completions_client.py tests/test_client_multimodal_types.py verifiers/init.py
• uv run ty check verifiers/clients/openai_chat_completions_client.py
• uv run pytest tests/test_client_multimodal_types.py -q

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Note

Low Risk
Narrow prompt-serialization change with a targeted regression test; no auth, data, or API surface changes beyond message shaping.

Overview
Fixes replay of reasoning-only assistant turns when building OpenAI-compatible chat completion prompts: if content is missing and there are no tool calls but reasoning_content is set, the client now sends content="" instead of leaving content unset, while still passing through reasoning_content.

Adds a regression test in test_client_multimodal_types.py and bumps the package to 0.1.15.dev15.

^{Reviewed by Cursor Bugbot for commit 9c167f9. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Serialize reasoning-only assistant messages with empty string content in to_native_prompt

When an AssistantMessage has content=None, no tool calls, and reasoning_content set, openai_chat_completions_client.py previously passed None as the content field. It now passes an empty string instead, satisfying the OpenAI API requirement that assistant messages with reasoning content have a non-null content value.

^{Macroscope summarized 9c167f9.}

[willccbb]

Closing per maintainer direction: this touches verifiers core, and this task should stay isolated to tasksets.