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

coderd/apidoc/docs.go

@@ -17122,7 +17122,8 @@ const docTemplate = `{
                 "config",
                 "usage_limit",
                 "missing_key",
-                "provider_disabled"
+                "provider_disabled",
+                "content_filter"
             ],
             "x-enum-varnames": [
                 "ChatErrorKindGeneric",
@@ -17134,7 +17135,8 @@ const docTemplate = `{
                 "ChatErrorKindConfig",
                 "ChatErrorKindUsageLimit",
                 "ChatErrorKindMissingKey",
-                "ChatErrorKindProviderDisabled"
+                "ChatErrorKindProviderDisabled",
+                "ChatErrorKindContentFilter"
             ]
         },
         "codersdk.ChatFileMetadata": {

coderd/apidoc/swagger.json

@@ -15396,7 +15396,8 @@
 				"config",
 				"usage_limit",
 				"missing_key",
-				"provider_disabled"
+				"provider_disabled",
+				"content_filter"
 			],
 			"x-enum-varnames": [
 				"ChatErrorKindGeneric",
@@ -15408,7 +15409,8 @@
 				"ChatErrorKindConfig",
 				"ChatErrorKindUsageLimit",
 				"ChatErrorKindMissingKey",
-				"ChatErrorKindProviderDisabled"
+				"ChatErrorKindProviderDisabled",
+				"ChatErrorKindContentFilter"
 			]
 		},
 		"codersdk.ChatFileMetadata": {