feat: record and expose terminal upstream interception errors (#26961)

Categorises the terminal error of a failed interception and persists it
on the interception record, then surfaces it on the AI Gateway API.

- Categorise into an enum (`bad_request`, `unauthorized`,
  `rate_limited`, `overloaded`, `server_error`, `unknown`), unwrapping
  the ResponseError envelope, the upstream Anthropic/OpenAI SDK errors,
  and key-pool exhaustion so blocking and streaming paths agree.
- Thread the type and raw message through the recorder dRPC into the
  `aibridge_interceptions` row (optional proto fields; NULL on success).
- Expose the error on the AI Gateway thread API from the root
  interception.

*This PR was produced by opencode (agent) using the `anthropic/claude-opus-4-8` model, under human direction and review.*

coderd/apidoc/docs.go

@@ -15227,6 +15227,14 @@ const docTemplate = `{
                     "type": "string",
                     "format": "date-time"
                 },
+                "error_message": {
+                    "description": "ErrorMessage is the raw terminal upstream error message from the root\ninterception. Nil when the interception succeeded.",
+                    "type": "string"
+                },
+                "error_type": {
+                    "description": "ErrorType is the categorized terminal upstream error from the root\ninterception, or nil when the interception succeeded. See the\naibridge_interception_error_type enum for possible values.",
+                    "type": "string"
+                },
                 "id": {
                     "type": "string",
                     "format": "uuid"

coderd/apidoc/swagger.json

@@ -13569,6 +13569,14 @@
 					"type": "string",
 					"format": "date-time"
 				},
+				"error_message": {
+					"description": "ErrorMessage is the raw terminal upstream error message from the root\ninterception. Nil when the interception succeeded.",
+					"type": "string"
+				},
+				"error_type": {
+					"description": "ErrorType is the categorized terminal upstream error from the root\ninterception, or nil when the interception succeeded. See the\naibridge_interception_error_type enum for possible values.",
+					"type": "string"
+				},
 				"id": {
 					"type": "string",
 					"format": "uuid"