fix: preserve reasoning_content for DeepSeek chat completions

[lizzielizzielizzie]

DeepSeek V4 enables thinking mode by default and requires that the reasoning_content field be passed back unchanged in assistant messages during multi-turn conversations. The openai-chat backend, however, does not support this. This PR adds that support, roughly following the pattern that is employed by the anthropic backend.

Fixes #3266

Flow

reasoning-start, delta, and end SSE events are captured, sent to the frontend, and included in subsequent API calls

When a stream chunk contains reasoning_content in its delta, we now:

1. Emit reasoning-start / reasoning-delta / reasoning-end SSE events to the frontend
2. Capture the full reasoning string on the stored assistant message
3. Round-trip it in subsequent API calls via reasoning_content on the wire format

Non-reasoning providers are unaffected — the stream field is absent, the Go field defaults to "", and the omitempty JSON tag keeps it off the wire.

Automated Testing

• tests for round-trip, omitempty, stream chunk parsing, clean, and partial extraction

Manual Testing

• I manually validated that the issue was resolved
• I validated that the three other chat modes that come with Wave still work as expected
• I validated that tool calls work
• I did not validate any other openai-style custom chat providers other than deepseek
  • Sorry, I don't have an API key for this 😔
  • I believe the changes are well-guarded, but it seems worth calling out as an area for extra attention

Is there anything else I should cover?

Screenshots

Before screenshot, demonstrating issue reproduction

Afterscreenshot, demonstrating issue resolution

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All committers have signed the CLA.

[coderabbitai]

Walkthrough

The diff adds separate tracking for streaming "reasoning" deltas: ChatRequestMessage and ContentDelta gain a ReasoningContent field (with JSON wire mapping). processChatStream accumulates reasoning, emits SSE lifecycle events (AiMsgReasoningStart/AiMsgReasoningDelta/AiMsgReasoningEnd), ensures reasoning is ended if necessary, and includes reasoning in the final assistant message. extractPartialTextMessage now accepts both text and reasoning and returns nil only when both are empty. Tests were added to validate JSON round-trips, streaming chunk parsing, clean() behavior, and partial-message extraction involving reasoning.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and accurately summarizes the main change: adding support for preserving reasoning_content in DeepSeek chat completions.
Linked Issues check	✅ Passed	The PR fully addresses issue #3266 by capturing reasoning-start/delta/end events, storing reasoning on assistant messages, and round-tripping reasoning_content in API calls.
Out of Scope Changes check	✅ Passed	All changes are scoped to adding reasoning_content support in the openai-chat backend; no extraneous modifications were introduced outside the stated objectives.
Description check	✅ Passed	The PR description clearly relates to the changeset, detailing how reasoning_content support was added to the openai-chat backend to fix DeepSeek V4 multi-turn conversations.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@pkg/aiusechat/openaichat/openaichat-backend.go`:
- Around line 264-269: Currently the code waits until after the generation loop
to call sseHandler.AiMsgReasoningEnd(msgID), causing reasoning-end to be emitted
after AiMsgTextEnd; modify the loop that switches from reasoning deltas to text
deltas to detect the reasoning→text transition, call
sseHandler.AiMsgReasoningEnd(msgID) immediately at that point and set
reasoningStarted = false (so the post-loop block won't emit it again), ensuring
the existing post-loop branch still covers the reasoning-only case; update
references: reasoningStarted, textStarted, sseHandler.AiMsgReasoningEnd(msgID),
sseHandler.AiMsgTextEnd(textID).

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Fix all unresolved CodeRabbit comments on this PR:

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: b1e3572d-e024-4956-9e24-e0184e214682

📥 Commits

Reviewing files that changed from the base of the PR and between 021db67 and 77d0cf1.

📒 Files selected for processing (3)

• pkg/aiusechat/openaichat/openaichat-backend.go
• pkg/aiusechat/openaichat/openaichat-backend_test.go
• pkg/aiusechat/openaichat/openaichat-types.go