Fix missing 'Preview' designation in gemini-3-pro-preview model label

The MODEL_CATALOG entry for gemini-3-pro-preview had its label set to
"Gemini 3 Pro", omitting the "Preview" designation and making it appear
as a stable GA model. The sibling entry gemini-3-flash-preview correctly
uses "Gemini 3 Flash Preview". This patch aligns the label with the model
alias and its preview status.

https://claude.ai/code/session_01E5pJiQ6Cu726PCBx9cJ76X

Author: claude

src/dumpcode/ai/models.py

@@ -47,8 +47,8 @@ class ModelConfig(TypedDict):
     # --- Google ---
     # Google uses its own SDK methods, so this param is informative/unused for now but good for consistency
     "gemini-3-pro-preview": {
-        "provider": "google", "model_id": "gemini-3-pro-preview", "context": 1048576, "label": "Gemini 3 Pro",
-        "token_param": "max_output_tokens" 
+        "provider": "google", "model_id": "gemini-3-pro-preview", "context": 1048576, "label": "Gemini 3 Pro Preview",
+        "token_param": "max_output_tokens"
     },
     "gemini-2.5-pro": {
         "provider": "google", "model_id": "gemini-2.5-pro", "context": 1048576, "label": "Gemini 2.5 Pro",