feat: surface model content-filter refusals as a blocked chat error (#27118)

Blocked turns from a provider's content filter (Anthropic's `refusal`
stop reason with empty content) previously ended silently on the
"Thinking" spinner. They now end as a terminal `content_filter` error
that renders as a "Response blocked" message with the provider's
category and explanation.

<img width="888" height="335" alt="image"
src="https://github.com/user-attachments/assets/cef85a59-4091-4e62-9d45-1eb06748db48"
/>


Closes CODAGT-611

Follow-ups will involve implementing fallbacks, but this alone is pretty
important

codersdk/chats.go

@@ -1715,6 +1715,7 @@ const (
 	ChatErrorKindUsageLimit           ChatErrorKind = "usage_limit"
 	ChatErrorKindMissingKey           ChatErrorKind = "missing_key"
 	ChatErrorKindProviderDisabled     ChatErrorKind = "provider_disabled"
+	ChatErrorKindContentFilter        ChatErrorKind = "content_filter"
 )
 
 // AllChatErrorKinds contains every ChatErrorKind value.
@@ -1730,6 +1731,7 @@ var AllChatErrorKinds = []ChatErrorKind{
 	ChatErrorKindUsageLimit,
 	ChatErrorKindMissingKey,
 	ChatErrorKindProviderDisabled,
+	ChatErrorKindContentFilter,
 }
 
 // ChatError represents a terminal chat error in persisted chat state or the