fix: include reasoning_content field for DeepSeek Reasoner models

[mvanhorn]

Summary

• DeepSeek Reasoner API requires reasoning_content on every assistant message, but appendReasoningFieldsIfSupported returned early when no thinking message preceded the assistant message (e.g. in resumed sessions)
• Now reasoning_content defaults to an empty string when includeReasoningContentField is true but no thinking content is available, preventing 400 "Missing reasoning_content field" errors

How it works

• Moved the includeReasoningContent check before the early return guard so it always runs for providers that need it
• When a thinking message precedes the assistant message, the actual reasoning content is used
• When no thinking message is present, an empty string is used as the default
• Other reasoning fields (reasoning, reasoning_details) still require a preceding thinking message

Test plan

• Verify DeepSeek Reasoner no longer returns 400 errors in resumed sessions
• Verify DeepSeek Reasoner still includes actual reasoning_content when thinking messages are present
• Verify Claude and other providers are unaffected (they use reasoning_details, not reasoning_content)

Fixes #11285

🤖 Generated with Claude Code

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Summary by cubic

Always include reasoning_content for DeepSeek Reasoner models, defaulting to an empty string when no prior thinking message exists. Prevents 400 errors in resumed sessions and keeps other providers unchanged.

• Bug Fixes
  • Run includeReasoningContentField logic before the early return and set msg.reasoning_content to the previous thinking content or "" if missing.
  • Leave reasoning and reasoning_details unchanged and only set when a thinking message is present.

^{Written for commit a843400. Summary will update on new commits.}

[mvanhorn]

I have read the CLA Document and I hereby sign the CLA

[cubic-dev-ai]

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