fix(@effect/ai-openai): map max_output_tokens finish reason to length (#6297)

Author: shtse8

.changeset/openai-max-output-tokens-finish-reason.md

@@ -0,0 +1,5 @@
+---
+"@effect/ai-openai": patch
+---
+
+Map the OpenAI Responses `max_output_tokens` finish reason to `length` instead of falling through to `unknown`.

packages/ai/openai/src/internal/utilities.ts

@@ -11,6 +11,7 @@ const finishReasonMap: Record<string, Response.FinishReason> = {
   content_filter: "content-filter",
   function_call: "tool-calls",
   length: "length",
+  max_output_tokens: "length",
   stop: "stop",
   tool_calls: "tool-calls"
 }