Fix reasoning summary part boundaries

[XiaoConstantine]

Motivation

OpenAI Responses streams reasoning summaries as multiple parts. Tau consumed each response.reasoning_summary_text.delta event but ignored response.reasoning_summary_part.done, causing adjacent Markdown headings to be persisted and rendered without separators, for example **First****Second**.

Changes

• Preserve reasoning-summary part boundaries as blank lines in the OpenAI Codex provider.
• Apply the same behavior to the OpenAI-compatible Responses parser.
• Add deterministic SSE regression tests covering streamed deltas and finalized thinking content for both provider paths.

This matches Pi's Responses stream handling at the provider boundary.

Testing

• env -u OPENAI_API_KEY uv run pytest -q — 1121 passed
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy
• git diff --check

Compatibility

No configuration or migration changes. Existing persisted reasoning text is unchanged; newly streamed summaries preserve part boundaries.