fix: update fantasy to fix OpenAI reasoning replay with Store enabled (#23297)

## Problem

When `Store: true` is set for OpenAI Responses API calls (the new
default), multi-turn conversations with reasoning models fail on the
second message:

```
stream response: bad request: Item 'rs_xxx' of type 'reasoning' was provided
without its required following item.
```

The fantasy library was reconstructing full `OfReasoning` input items
(with encrypted content and summary) when replaying assistant messages.
The API cannot pair these reconstructed reasoning items with the output
items that originally followed them because the output items are sent as
plain `OfMessage` without server-side IDs.

## Fix

Updates the fantasy dependency (`kylecarbs/fantasy@cj/go1.25`) to skip
reasoning parts during conversation replay in `toResponsesPrompt`. With
`Store` enabled, the API already has the reasoning persisted server-side
— it doesn't need to be replayed in the input.

Fantasy PR: https://github.com/charmbracelet/fantasy/pull/181

## Testing

Adds `TestOpenAIReasoningRoundTrip` integration test that:
1. Sends a query to `o4-mini` (reasoning model with `Store: true`)
2. Verifies reasoning content is persisted
3. Sends a follow-up message — this was the failing step
4. Verifies the follow-up completes successfully

Requires `OPENAI_API_KEY` env var to run.

go.mod

@@ -80,7 +80,7 @@ replace github.com/spf13/afero => github.com/aslilac/afero v0.0.0-20250403163713
 // 1) Adds thinking effort to Anthropic provider
 // 2) Downgraded to Go 1.25 due to issue with Windows CI
 // https://github.com/kylecarbs/fantasy/compare/main...kylecarbs:fantasy:cj/go1.25
-replace charm.land/fantasy => github.com/kylecarbs/fantasy v0.0.0-20260319114014-12345ae15482
+replace charm.land/fantasy => github.com/kylecarbs/fantasy v0.0.0-20260319151840-18e18e661ed4
 
 replace github.com/charmbracelet/anthropic-sdk-go => github.com/kylecarbs/anthropic-sdk-go v0.0.0-20260223140439-63879b0b8dab