fix(cli): show reasoning without streaming

[LeonSGP43]

Summary

• make CLI show_reasoning work even when streaming: false by routing non-streaming reasoning through the preview callback
• mark previewed reasoning as already shown so the final response path does not render a duplicate reasoning box
• update CLI regression tests for non-streaming reasoning display selection and preview de-duplication

Closes #34209.

Verification

• uv run --frozen pytest -q -o addopts='' tests/cli/test_reasoning_command.py -k 'ReasoningPreviewBuffering or ReasoningDisplayModeSelection'
• uv run --frozen ruff check cli.py tests/cli/test_reasoning_command.py
• git diff --check

[teknium1]

Thanks for the focused fix. Current main still returns no reasoning callback for show_reasoning=True with streaming_enabled=False (cli.py:5412-5418), while non-streaming assistant-message processing invokes a registered reasoning callback (agent/chat_completion_helpers.py:1060-1071). Routing this case to _on_reasoning therefore uses the existing buffered preview path (cli.py:9434-9439).

The added preview flag also aligns with the final-response guard at cli.py:12640-12642, preventing a rendered preview from being followed by a duplicate final reasoning box. The duplicate PR #34355 was closed in favor of this PR and explicitly noted that safeguard.

Automated hermes-sweeper review.