fix: handle OpenAI Responses API stream errors instead of crashing (#1669)

The streaming code assumed every stream produced a `response.completed`
event and dereferenced its data unconditionally, causing
`undefined method 'data' for nil` whenever OpenAI emitted
`response.failed`, `response.incomplete`, or a top-level `error` event
(e.g. expired `previous_response_id`, context-window overflow,
transient upstream failures). Surface a descriptive `Provider::Error`
instead.

- Extend `ChatStreamParser` to recognise `response.failed`,
  `response.incomplete`, and `error` events and emit an `error` chunk
  with a `StreamErrorData` payload (event, message, code, details).
- In `Provider::Openai#native_chat_response`, detect the missing
  `response` chunk, build a user-facing error message from the
  collected error chunk, and raise `Provider::Error`.
- Add unit tests for the parser (8 cases) and integration tests for
  the error path in the chat response flow.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/models/provider/openai/chat_stream_parser.rb

@@ -1,6 +1,8 @@
 class Provider::Openai::ChatStreamParser
   Error = Class.new(StandardError)
 
+  StreamErrorData = Data.define(:event, :message, :code, :details)
+
   def initialize(object)
     @object = object
   end
@@ -15,6 +17,21 @@ class Provider::Openai::ChatStreamParser
       raw_response = object.dig("response")
       usage = raw_response.dig("usage")
       Chunk.new(type: "response", data: parse_response(raw_response), usage: usage)
+    when "response.failed"
+      Chunk.new(type: "error", data: build_response_error("response.failed"), usage: nil)
+    when "response.incomplete"
+      Chunk.new(type: "error", data: build_response_error("response.incomplete"), usage: nil)
+    when "error"
+      Chunk.new(
+        type: "error",
+        data: StreamErrorData.new(
+          event: "error",
+          message: object.dig("message").presence || "OpenAI stream returned an error event",
+          code: object.dig("code"),
+          details: object
+        ),
+        usage: nil
+      )
     end
   end
 
@@ -26,4 +43,22 @@ class Provider::Openai::ChatStreamParser
     def parse_response(response)
       Provider::Openai::ChatParser.new(response).parsed
     end
+
+    def build_response_error(event)
+      raw_response = object.dig("response") || {}
+      error_message =
+        raw_response.dig("error", "message").presence ||
+        raw_response.dig("incomplete_details", "reason").presence ||
+        "OpenAI stream ended with #{event}"
+      code =
+        raw_response.dig("error", "code") ||
+        raw_response.dig("incomplete_details", "reason")
+
+      StreamErrorData.new(
+        event: event,
+        message: error_message,
+        code: code,
+        details: raw_response
+      )
+    end
 end