chore: update fantasy dep to rebased cj/go1.25 branch (#23242)

Updates the `charm.land/fantasy` replace to the rebased `cj/go1.25`
branch on `kylecarbs/fantasy`, which now includes:

- **chore: downgrade to Go 1.25**
- **feat: anthropic computer use**
- **chore: use kylecarbs/openai-go fork for coder/coder compat**

Switches the `openai-go/v3` replace from `SasSwart/openai-go` →
`kylecarbs/openai-go`, which is the same SasSwart perf fork plus a fix
for `WithJSONSet` being clobbered by deferred body serialization.
Without the fix, `NewStreaming` silently drops `stream: true` from
requests. See https://github.com/kylecarbs/openai-go/pull/2 for details.

coderd/chatd/chatprovider/chatprovider.go

@@ -1083,6 +1083,7 @@ func openAIProviderOptionsFromChatConfig(
 			SafetyIdentifier:  normalizedStringPointer(options.SafetyIdentifier),
 			ServiceTier:       openAIServiceTierFromChat(options.ServiceTier),
 			StrictJSONSchema:  options.StrictJSONSchema,
+			Store:             boolPtrOrDefault(options.Store, true),
 			TextVerbosity:     OpenAITextVerbosityFromChat(options.TextVerbosity),
 			User:              normalizedStringPointer(options.User),
 		}
@@ -1099,7 +1100,7 @@ func openAIProviderOptionsFromChatConfig(
 		MaxCompletionTokens: options.MaxCompletionTokens,
 		TextVerbosity:       normalizedStringPointer(options.TextVerbosity),
 		Prediction:          options.Prediction,
-		Store:               options.Store,
+		Store:               boolPtrOrDefault(options.Store, true),
 		Metadata:            options.Metadata,
 		PromptCacheKey:      normalizedStringPointer(options.PromptCacheKey),
 		SafetyIdentifier:    normalizedStringPointer(options.SafetyIdentifier),
@@ -1280,6 +1281,13 @@ func useOpenAIResponsesOptions(model fantasy.LanguageModel) bool {
 	}
 }
 
+func boolPtrOrDefault(value *bool, def bool) *bool {
+	if value != nil {
+		return value
+	}
+	return &def
+}
+
 func normalizedStringPointer(value *string) *string {
 	if value == nil {
 		return nil