chore: bump coder/fantasy to include gpt-5.5 Responses API support (#24712)

Bumps the `charm.land/fantasy` replace directive to pick up
<https://github.com/coder/fantasy/pull/29>, which adds `gpt-5.5` and
`gpt-5.5-pro` to `responsesReasoningModelIDs`.

Without this, chatd's `useOpenAIResponsesOptions` returns false for
GPT-5.5, so it falls back to Chat Completions and never attaches
`ResponsesProviderOptions` (losing `store=true` + `previous_response_id`
chaining and other Responses-only features).

## Changes
- `go.mod`: `github.com/coder/fantasy
v0.0.0-20260416152503-959aa39579d2` →
`v0.0.0-20260424191546-5ab464a305f4`
- `go.sum`: updated hashes

Verified `go build ./coderd/x/chatd/...` passes locally.

Created on behalf of @ibetitsmike

Co-authored-by: blink-so[bot] <211532188+blink-so[bot]@users.noreply.github.com>

go.mod

@@ -83,8 +83,8 @@ replace github.com/spf13/afero => github.com/aslilac/afero v0.0.0-20250403163713
 // 4) (anthropic-sdk-go) dannykopping's appendCompact performance fixes
 // 5) (anthropic-sdk-go) DirectEncoder to eliminate nested MarshalJSON allocation chain
 // 6) Anthropic EffortXHigh constant for Claude Opus 4.7
-// See: https://github.com/coder/fantasy/commits/959aa39579d2
-replace charm.land/fantasy => github.com/coder/fantasy v0.0.0-20260416152503-959aa39579d2
+// See: https://github.com/coder/fantasy/commits/5ab464a305f4
+replace charm.land/fantasy => github.com/coder/fantasy v0.0.0-20260424191546-5ab464a305f4
 
 // coder/coder uses a fork of charmbracelet's fork of the Anthropic Go SDK with some
 // additional performance improvements.