Add support for Gemini 2.5 flash stable (#6692)

As Gemini 2.5 Flash was released as stable the model infos should be
changed accordingly.

See https://ai.google.dev/gemini-api/docs/models?hl=de#gemini-2.5-flash

## Related issue number

No issue

## Checks

- [x] I've included any doc changes needed for
<https://microsoft.github.io/autogen/>. See
<https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md> to
build and test documentation locally.
- [x] I've added tests (if relevant) corresponding to the changes
introduced in this PR.
- [x] I've made sure all auto checks have passed.

---------

Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>

python/packages/autogen-core/src/autogen_core/models/_model_client.py

@@ -67,7 +67,7 @@ class ModelFamily:
         "gemini-1.5-pro",
         "gemini-2.0-flash",
         "gemini-2.5-pro",
-        "gemini-2.5-flash"
+        "gemini-2.5-flash",
         # anthropic_models
         "claude-3-haiku",
         "claude-3-sonnet",

python/packages/autogen-ext/src/autogen_ext/models/openai/_model_info.py

@@ -310,7 +310,7 @@ _MODEL_INFO: Dict[str, ModelInfo] = {
         "structured_output": True,
         "multiple_system_messages": False,
     },
-    "gemini-2.5-flash-preview-05-20": {
+    "gemini-2.5-flash": {
         "vision": True,
         "function_calling": True,
         "json_output": True,
@@ -448,7 +448,7 @@ _MODEL_TOKEN_LIMITS: Dict[str, int] = {
     "gemini-2.0-flash": 1048576,
     "gemini-2.0-flash-lite-preview-02-05": 1048576,
     "gemini-2.5-pro-preview-03-25": 2097152,
-    "gemini-2.5-flash-preview-05-20": 1048576,
+    "gemini-2.5-flash": 1048576,
     "claude-3-haiku-20240307": 50000,
     "claude-3-sonnet-20240229": 200000,
     "claude-3-opus-20240229": 200000,