fix: deprecate old GeminiImage node (gemini-2.5-flash-image-preview is gone)

Google removed the gemini-2.5-flash-image-preview model from Vertex AI.
The proxy sanitizes existing requests to the GA model, and the older
GeminiImage node is now marked is_deprecated=True so users are steered
toward GeminiImage2 which uses current models.

comfy_api_nodes/nodes_gemini.py

@@ -644,6 +644,7 @@ class GeminiImage(IO.ComfyNode):
                 IO.Hidden.unique_id,
             ],
             is_api_node=True,
+            is_deprecated=True,
             price_badge=IO.PriceBadge(
                 expr="""{"type":"usd","usd":0.039,"format":{"suffix":"/Image (1K)","approximate":true}}""",
             ),