feat: add chatgpt support for aibridge (#23822)

Registers a new aibridge provider for ChatGPT by reusing the existing
OpenAI provider with a different `Name` and `BaseURL`
(https://chatgpt.com/backend-api/codex). The ChatGPT backend API is
OpenAI-compatible, so no new provider type is needed.
ChatGPT authenticates exclusively via per-user OAuth JWTs (BYOK mode) —
no centralized API key is configured. The OpenAI provider already
handles this: when no key is set, it falls through to the bearer token
from the request's Authorization header.
  
  Depends on #23811

coderd/aibridge/aibridge.go

@@ -28,6 +28,12 @@ const (
 	HostCopilotEnterprise     = "api.enterprise.githubcopilot.com"
 )
 
+// ChatGPT provider.
+const (
+	ProviderChatGPT = "chatgpt"
+	BaseURLChatGPT  = "https://chatgpt.com/backend-api/codex"
+)
+
 // IsBYOK reports whether the request is using BYOK mode, determined
 // by the presence of the X-Coder-AI-Governance-Token header.
 func IsBYOK(header http.Header) bool {