gptel-openai: Refine reasoning block detection in stream parser

* gptel-openai.el (gptel-curl--parse-stream): Previously the
stream parser assumed that all streaming chunks in a response from
a streaming model would have the :reasoning field, even if its
value was null.  Post-reasoning content blocks would then have
a (possibly null) :reasoning field and a non-null :content field.
Reasoning block processing depended partially on this assumption.

Use only the value of :reasoning-block instead.  This
implies a little extra work on the very first streaming chunk, but
makes fewer assumptions about the presence of both :reasoning and
:content fields in all future chunks. (#901)

Author: karthink

gptel-openai.el

@@ -231,9 +231,9 @@ information if the stream contains it."
                       ;; old tool block continues, so continue collecting arguments in :partial_json 
                       (push (plist-get func :arguments) (plist-get info :partial_json)))))
                 ;; Check for reasoning blocks, currently only used by Openrouter
-                ;; MAYBE: Should this be moved to a dedicated Openrouter backend?
-                (unless (or (eq (plist-get info :reasoning-block) 'done)
-                            (not (plist-member delta :reasoning)))
+                (unless (eq (plist-get info :reasoning-block) 'done)
+                  ;; We could also check for (plist-member delta :reasoning)
+                  ;; here, but some APIs don't include it with content. (#901)
                   (if-let* ((reasoning-chunk (plist-get delta :reasoning)) ;for openrouter
                             ((not (eq reasoning-chunk :null))))
                       (plist-put info :reasoning