fix(protocols): always serialize chat message content (null when absent)

[rmccorm4]

Summary

Non-streaming /v1/chat/completions responses were dropping the content key entirely from the JSON message any time content was None (reasoning-only responses, tool-call-only responses). This broke clients like the one in DGH-651 that read response.json()['choices'][0]['message']['content'].

Root cause: ChatCompletionResponseMessage.content carried #[serde(skip_serializing_if = "Option::is_none")], so None was omitted instead of serialized as null. Upstream OpenAI's API always emits the content key (as null when there is no content) alongside reasoning_content or tool_calls.

Fix: remove skip_serializing_if from that one field. Now content: null is always emitted when there's no text/content-parts, matching OpenAI's wire format.

Approach history:

• First commit attempted a narrower workaround in the aggregator (set content to Some(Text("")) when reasoning_content was present). That was a semantic muddle ("no content" vs "empty content") and only fixed the reasoning case, not the tool-call-only case.
• Second commit replaces it with the serialization-level fix, which is simpler (1 attribute removed) and covers both shapes.

Fixes: DGH-651
Closes #7154

Test plan

• New unit test test_reasoning_only_response_serializes_content_key_as_null asserts content serializes as serde_json::Value::Null when reasoning-only.
• Empirically verified the test fails on main without the fix, passes with it.
• Full workspace test suite: 2000+ tests, 0 failures.

```
cargo test --workspace --lib

all passing

```

🤖 Generated with Claude Code

[devin-ai-integration]

Devin Review found 0 potential issues.

View 1 additional finding in Devin Review.

[coderabbitai]

Walkthrough

Modified the From<DeltaChoice> implementation in the OpenAI chat completions aggregator to ensure the content field is retained in the message response when only reasoning_content is present, preventing the field from being omitted during serialization.

Changes

Cohort / File(s)	Summary
Protocol Aggregator lib/llm/src/protocols/openai/chat_completions/aggregator.rs	Updated delta choice conversion to retain content as an empty string when reasoning_content exists but text and content\_parts are absent; added unit test validating both fields serialize correctly.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The code changes directly address the issue #7154 by ensuring both content and reasoning_content fields are present in the JSON response, matching OpenAI convention.
Out of Scope Changes check	✅ Passed	All changes are focused on fixing the aggregator conversion logic and adding relevant unit tests; no unrelated modifications detected.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Title check	✅ Passed	The title accurately summarizes the main change: ensuring chat message content is always serialized, including null values when absent, which directly matches the fix implementation.
Description check	✅ Passed	The PR description is comprehensive and well-structured, covering overview, detailed explanation of root cause, fix approach, and test plan with command verification.

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