## Problem

Users are experiencing intermittent OpenAI API errors when using
reasoning models with tool calls:
- `Item 'rs_*' of type 'reasoning' was provided without its required
following item`
- `referenced reasoning on a function_call was not provided`

The previous fix (PR #61) stripped reasoning parts entirely, but this
caused new errors and was too aggressive.

## Root Cause

OpenAI's Responses API uses encrypted reasoning items (IDs like `rs_*`)
that are managed automatically via `previous_response_id`. When provider
metadata from stored history is sent back to OpenAI, it references
reasoning items that no longer exist in the current context, causing API
errors.

## Solution

Instead of stripping reasoning content, we now **blank out provider
metadata** on all content parts for OpenAI:
- Clear `providerMetadata` on text and reasoning parts  
- Clear `callProviderMetadata` on tool-call parts

This preserves the reasoning content (which is useful for debugging and
context) while preventing stale metadata references from causing errors.

## Changes

1. **New function**: `clearProviderMetadataForOpenAI()` - operates on
`ModelMessage[]
3. **Fixed**: `splitMixedContentMessages()` now treats reasoning parts
as text parts (they stay together)
4. **Updated**: Tests to reflect that reasoning parts are preserved, not
stripped

## References

- Vercel AI SDK Issue: vercel/ai#7099
- User solution:
https://github.com/gvkhna/vibescraper/blob/f476c768266385affec3b5972790ef7b111da366/packages/website/src/assistant-ai/assistant-prepare-context.ts#L104

## Testing

- ✅ All message transform tests passing
- ✅ Reasoning parts preserved in both OpenAI and Anthropic flows
- ✅ Tool calls work correctly with reasoning
- ✅ Formatting checks pass

This PR fixes the intermittent OpenAI API error using Vercel AI SDK's
middleware pattern to intercept and transform messages before
transmission.

## Problem
OpenAI's Responses API intermittently returns this error during
streaming:
```
Item 'rs_*' of type 'reasoning' was provided without its required following item
```

The error occurs during **multi-step tool execution** when:
- Model generates reasoning + tool calls
- SDK automatically executes tools and prepares next step
- Tool-call parts contain OpenAI item IDs that reference reasoning items
- When reasoning is stripped but tool-call IDs remain, OpenAI rejects
the malformed input

## Root Cause
OpenAI's Responses API uses internal item IDs (stored in
`providerOptions.openai.itemId`) to link:
- Reasoning items (`rs_*`)
- Function call items (`fc_*`)

When the SDK reconstructs conversation history for multi-step execution:
1. Assistant message includes `[reasoning, tool-call]` parts
2. Tool-call has `providerOptions.openai.itemId: "fc_*"` referencing
`rs_*`
3. Previous middleware stripped reasoning but left tool-call with
dangling reference
4. OpenAI API rejects: "function_call fc_* was provided without its
required reasoning item rs_*"

## Solution
Enhanced **OpenAI reasoning middleware** to strip item IDs when removing
reasoning:

**File: `src/utils/ai/openaiReasoningMiddleware.ts`**
1. Detects assistant messages containing reasoning parts
2. Filters out reasoning parts (OpenAI manages via `previousResponseId`)
3. **NEW:** Strips `providerOptions.openai` from remaining parts to
remove item IDs
4. Prevents dangling references that cause API errors

**Applied in: `src/services/aiService.ts`**
- Wraps OpenAI models with `wrapLanguageModel({ model, middleware })`
- Middleware intercepts messages before API transmission
- Only affects OpenAI (not Anthropic or other providers)

## Testing Results
Tested against real chat history that reliably reproduced the error:

✅ **Turn 1: PASSED** - Previously failed 100% of the time, now succeeds
✅ **Turn 2: PASSED** - Multi-step tool execution works correctly  

The middleware successfully:
- Stripped 15 OpenAI item IDs from tool-call parts (Turn 1)
- Stripped 15 OpenAI item IDs from tool-call parts (Turn 2)  
- Allowed multi-step tool execution without reasoning errors

## Technical Details
**Multi-step execution flow:**
1. User sends message
2. Model generates reasoning + tool calls (Step 1)
3. SDK auto-executes tools
4. SDK prepares Step 2 input: `[system, user,
assistant(reasoning+tools), tool-results]`
5. Middleware strips reasoning + item IDs before sending
6. Step 2 proceeds without API errors

**Why this fixes it:**
- OpenAI Responses API validates item ID references on input
- Removing `providerOptions.openai.itemId` prevents validation errors
- OpenAI tracks context via `previousResponseId`, not message content
- SDK's automatic tool execution works correctly with cleaned messages

## Files Changed
- `src/services/aiService.ts`: Apply middleware to OpenAI models (7
lines)
- `src/utils/ai/openaiReasoningMiddleware.ts`: New middleware with item
ID stripping (112 lines)

## Related Issues
- Fixes OpenAI reasoning errors from vercel/ai SDK issues #7099, #8031,
#8977
- Supersedes previous approaches (PR #61, #68) that didn't use SDK
middleware

_Generated with `cmux`_