feat: add reasoning_content support for OpenRouter chat completions

[AlemTuzlak]

Summary

• Adds reasoning_content field to chat completions streaming (delta.reasoning_content) and non-streaming (message.reasoning_content) responses
• Updates collapseOpenAISSE to extract reasoning_content from chat completions delta chunks for record-and-replay
• OpenRouter models that support reasoning (DeepSeek, Claude via OpenRouter, etc.) return thinking tokens via this field — aimock previously only supported reasoning for the Responses API and Anthropic Messages API

What's included

This PR covers the aimock mock server side — emitting reasoning_content in /v1/chat/completions responses when a fixture has reasoning set. It does NOT cover any additional OpenRouter-specific behavior (e.g. reasoning_details, model routing, provider preferences). Those may need separate work.

Files changed

• src/types.ts — added reasoning_content to SSEDelta and ChatCompletionMessage
• src/helpers.ts — buildTextChunks and buildTextCompletion now accept and emit reasoning
• src/server.ts — passes response.reasoning through to chat completions builders
• src/stream-collapse.ts — extracts reasoning_content from chat completions deltas
• Tests — 5 new tests covering streaming, non-streaming, absence, delta reconstruction, and stream-collapse

Test plan

• Streaming: reasoning_content deltas emitted before content deltas
• Streaming: reasoning_content deltas reconstruct full reasoning text
• Streaming: content deltas still reconstruct full text
• Streaming: no reasoning_content when fixture has no reasoning
• Non-streaming: reasoning_content present in message
• Non-streaming: reasoning_content absent when no reasoning
• Stream-collapse: extracts reasoning_content from chat completions deltas
• All 2034 existing tests still pass

🤖 Generated with Claude Code

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commit: 66e61be

[jpr5]

PR Review Summary

Reviewed by 7 specialized agents (code-reviewer, silent-failure-hunter, test-analyzer, comment-analyzer, type-design-analyzer, test-file-reviewer, CI/packaging) across all 6 changed files.

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Zero actionable findings — clean PR.

The implementation is straightforward and well-executed. reasoning_content is correctly threaded through the full pipeline:

• Types (SSEDelta, ChatCompletionMessage): optional, not nullable — matches OpenRouter's wire format
• Helpers (buildTextChunks, buildTextCompletion): reasoning chunks emitted before content (streaming), conditional spread for non-streaming — both correct
• Server (handleCompletions): passes response.reasoning through to both builders — clean plumbing
• Stream collapse (collapseOpenAISSE): extracts reasoning_content from deltas using the same defensive pattern as existing content extraction

Tests are solid — 8 new tests covering streaming ordering, content/reasoning reconstruction, absence cases, and stream-collapse extraction. Assertions are specific (exact string equality, index comparison for ordering), not weak.

One note

The branch is ~21 commits behind main (branched before v1.8.0). A rebase before merge would confirm no conflicts with the reasoning work that landed in PR #81.

🤖 Reviewed with Claude Code

[jpr5]

Thank you! 🙏