fix: support xhigh reasoning effort for OpenAI models (#23545)

## Summary

Adds `xhigh` to the OpenAI reasoning effort normalizer so GPT-5.4 class
models can use `reasoning_effort: xhigh` without it being silently
dropped.

## Problem

The SDK schema (`codersdk/chats.go`) already advertises `xhigh` as a
valid `reasoning_effort` value, but the runtime normalizer in
`chatprovider.go` only accepts `minimal|low|medium|high` for the OpenAI
provider. When a user sets `xhigh`, `ReasoningEffortFromChat()` returns
`nil` and the value never reaches the OpenAI API.

## Changes

- **Fantasy dependency**: Updated `kylecarbs/fantasy` (cj/go1.25) which
now includes the `ReasoningEffortXHigh` constant
([kylecarbs/fantasy#9](https://github.com/kylecarbs/fantasy/pull/9)).
- **`chatprovider.go`**: Adds `fantasyopenai.ReasoningEffortXHigh` to
the OpenAI case in `ReasoningEffortFromChat()`.
- **`chatprovider_test.go`**: Adds `OpenAIXHighEffort` test case.

## Upstream

-
[charmbracelet/fantasy#186](https://github.com/charmbracelet/fantasy/pull/186)

coderd/x/chatd/chatprovider/chatprovider.go

@@ -487,6 +487,7 @@ func ReasoningEffortFromChat(provider string, value *string) *string {
 			string(fantasyopenai.ReasoningEffortLow),
 			string(fantasyopenai.ReasoningEffortMedium),
 			string(fantasyopenai.ReasoningEffortHigh),
+			string(fantasyopenai.ReasoningEffortXHigh),
 		)
 	case fantasyanthropic.Name:
 		return normalizedEnumValue(

coderd/x/chatd/chatprovider/chatprovider_test.go

@@ -29,6 +29,12 @@ func TestReasoningEffortFromChat(t *testing.T) {
 			input:    ptr.Ref(" HIGH "),
 			want:     ptr.Ref(string(fantasyopenai.ReasoningEffortHigh)),
 		},
+		{
+			name:     "OpenAIXHighEffort",
+			provider: "openai",
+			input:    ptr.Ref("xhigh"),
+			want:     ptr.Ref(string(fantasyopenai.ReasoningEffortXHigh)),
+		},
 		{
 			name:     "AnthropicEffort",
 			provider: "anthropic",