fix(translation): drop empty assistant messages in OpenAI Chat encoding

[Pana]

What

Use this project with moonshot kimi-k3 model, will encounter error: Invalid request: the message at position 5 with role 'assistant' must not be empty

Why

So this PR is used to improve compatibility to kimi API

Closes #

How tested

• uv run ruff check . clean
• uv run mypy switchyard clean
• uv run pytest tests/ green
• Manual smoke (describe what was run)

Checklist

• One class per file; filename = snake_case of the primary class.
• New public symbols exported from switchyard/__init__.py.__all__ if intended for downstream use.
• Unit tests added for new components / bug fixes.
• README / --help updated if customer-facing surface changed.
• Commits signed off (Signed-off-by: Your Name <email>) per the DCO.

Notes for reviewers

Anything reviewers should pay extra attention to — risky paths, follow-up tickets, intentional trade-offs.

Summary by CodeRabbit

• Bug Fixes
  • Improved OpenAI Chat request translation by omitting empty assistant messages that contain no tool calls.
  • Preserved assistant messages that contain tool calls, even when their text content is null.
  • Corrected handling of assistant turns containing only private thinking content while retaining surrounding user messages.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 32785209-1074-4cdf-b9c8-1f1673eacf82

📥 Commits

Reviewing files that changed from the base of the PR and between 2bef154 and b513d82.

📒 Files selected for processing (2)

• crates/switchyard-translation/src/codecs/openai_chat/buffered.rs
• crates/switchyard-translation/tests/request_translation.rs

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Walkthrough

The OpenAI Chat encoder now removes assistant messages with empty content when they have no tool calls. A regression test covers Anthropic assistant turns that contain only private thinking blocks.

Changes

OpenAI Chat message filtering

Layer / File(s)	Summary
Empty assistant message filtering crates/switchyard-translation/src/codecs/openai_chat/buffered.rs	The encoder removes assistant messages with empty-string content when they have no tool calls. Tool-call messages remain preserved.
Thinking-only turn regression coverage crates/switchyard-translation/tests/request_translation.rs	The test verifies that thinking-only Anthropic assistant turns are omitted and surrounding user messages remain intact.

Estimated code review effort: 2 (Simple) | ~10 minutes

Poem

I’m a rabbit with a tidy tray,
Empty assistant turns hop away.
Tool calls stay, both crisp and bright,
Thinking-only turns fade from sight.
Tests keep the message path just right.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: omitting empty assistant messages during OpenAI Chat encoding.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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_{Comment @coderabbitai help to get the list of available commands.}

[ayushag-nv]

@Pana Thanks for adding the fix. Can you please add the reproduction steps for the issue, and before the fix snapshot with error and after the fix snapshot.