Add model "learnlm-1.5-pro-experimental" (simonw#36)

See https://ai.google.dev/gemini-api/docs/models/experimental-models for all available experimental models. learnlm was missing from the list in the plugin.

https://ai.google.dev/gemini-api/docs/learnlm for more details.

Author: leotulipan

llm_gemini.py

@@ -41,6 +41,7 @@ def register_models(register):
         "gemini-exp-1121",
         "gemini-exp-1206",
         "gemini-2.0-flash-exp",
+        "learnlm-1.5-pro-experimental",
         "gemini-2.0-flash-thinking-exp-1219",
         "gemini-2.0-flash-thinking-exp-01-21",
     ]:
@@ -336,4 +337,4 @@ def embed_batch(self, items):
             )
 
         response.raise_for_status()
-        return [item["values"] for item in response.json()["embeddings"]]
+        return [item["values"] for item in response.json()["embeddings"]]