🤖 fix: preserve OpenAI reasoning providerMetadata for Responses API

[ibetitsmike]

Problem

When using OpenAI reasoning models (o1, o3-mini, etc.), mux was showing warnings:

AI SDK Warning: Non-OpenAI reasoning parts are not supported. Skipping reasoning part: {"type":"reasoning","text":"..."}

This occurred because:

1. OpenAI's Responses API returns reasoning parts with providerMetadata.openai.itemId which is required when sending reasoning parts back in subsequent requests. mux wasn't capturing this metadata.
2. When conversation history contained reasoning from non-OpenAI models (e.g., Claude extended thinking), those parts would cause warnings when sent to OpenAI since they don't have the required itemId.

Solution

Capture providerMetadata from OpenAI reasoning streams

• Add providerMetadata field to MuxReasoningPart type
• Update streamManager.ts to capture providerMetadata from reasoning-delta events
• Use SDK's delta field (not text) for reasoning content

Filter non-OpenAI reasoning before sending to OpenAI

• Add filterNonOpenAIReasoningParts() function that strips reasoning parts without providerOptions.openai.itemId
• Apply filter BEFORE coalescing to prevent mixing non-OpenAI reasoning with valid OpenAI reasoning

The AI SDK's convertToModelMessages() maps providerMetadata → providerOptions, so storing it on reasoning parts ensures proper round-tripping.

Testing

• All unit tests pass (1851 tests)
• Static checks pass
• Added 3 new tests for non-OpenAI reasoning filtering
• Updated existing tests to include required providerOptions.openai.itemId

---

Generated with mux • Model: claude-sonnet-4-20250514 • Thinking: low

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