refactor: type computer use provider as an enum (#27086)

The deployment-wide computer use provider was passed around as a bare `string` on the `codersdk` wire structs, in `chattool`, and in the generated TypeScript, and its valid values (`anthropic`, `openai`) were never exposed as a `codersdk` enum. That's out of step with our other chat settings (`ChatDebugRunKind`, `ChatUsageLimitPeriod`), which already define enums with `Valid()` and an `All<Name>s` slice, and it left the allowed values duplicated as literals with no typed contract for clients.

This adds `codersdk.ChatComputerUseProvider` as the single source of truth and routes the API boundary, `chattool`, `chatd`, and the generated TypeScript through it. The DB layer and chattool's internal model-provider routing stay `string` on purpose, since they handle untrusted or fantasy-model values that just happen to share the names.

codersdk/chats.go

@@ -913,16 +913,36 @@ type AdvisorConfig struct {
 // the request and response shapes are currently identical.
 type UpdateAdvisorConfigRequest = AdvisorConfig
 
+// ChatComputerUseProvider identifies the provider that backs computer use for
+// the virtual desktop.
+type ChatComputerUseProvider string
+
+const (
+	ChatComputerUseProviderAnthropic ChatComputerUseProvider = "anthropic"
+	ChatComputerUseProviderOpenAI    ChatComputerUseProvider = "openai"
+)
+
+// AllChatComputerUseProviders contains every ChatComputerUseProvider value.
+var AllChatComputerUseProviders = []ChatComputerUseProvider{
+	ChatComputerUseProviderAnthropic,
+	ChatComputerUseProviderOpenAI,
+}
+
+// Valid reports whether p is a supported computer use provider.
+func (p ChatComputerUseProvider) Valid() bool {
+	return slices.Contains(AllChatComputerUseProviders, p)
+}
+
 // ChatComputerUseProviderResponse is the response for getting the computer use
 // provider setting.
 type ChatComputerUseProviderResponse struct {
-	Provider string `json:"provider"`
+	Provider ChatComputerUseProvider `json:"provider"`
 }
 
 // UpdateChatComputerUseProviderRequest is the request to update the computer use
 // provider setting.
 type UpdateChatComputerUseProviderRequest struct {
-	Provider string `json:"provider"`
+	Provider ChatComputerUseProvider `json:"provider"`
 }
 
 // ChatDebugLoggingAdminSettings describes the runtime admin setting