Validate llm_config passed to ConversableAgent

Based on microsoft#1522, this commit implements the additional validation checks in
`ConversableAgent.`

Add the following validation and `raise ValueError` if:

 - The `llm_config` is `None` (validated in `ConversableAgent`).
 - The `llm_config` has no `model` specified and `config_list` is empty
   (validated in `OpenAIWrapper`).
 - The `config_list` has at least one entry, but not all the entries have
   the `model` is specified (validated in `OpenAIWrapper`).

The rest of the changes are code churn to adjust or add the test cases.

test/agentchat/contrib/test_web_surfer.py

@@ -51,7 +51,9 @@ def test_web_surfer() -> None:
         mp.setenv("OPENAI_API_KEY", "mock")
         page_size = 4096
         web_surfer = WebSurferAgent(
-            "web_surfer", llm_config={"config_list": []}, browser_config={"viewport_size": page_size}
+            "web_surfer",
+            llm_config={"config_list": [{"model": "gpt-4", "api_key": ""}]},
+            browser_config={"viewport_size": page_size},
         )
 
         # Sneak a peak at the function map, allowing us to call the functions for testing here

test/agentchat/test_conversable_agent.py

@@ -781,10 +781,6 @@ def test_register_for_llm_without_LLM():
         assert e.args[0] == "Please specify the value for 'llm_config'."
 
 
-def test_register_for_llm_without_configuration_but_with_model_name():
-    ConversableAgent(name="agent", llm_config={"model": "gpt-4", "config_list": []})
-
-
 def test_register_for_llm_without_configuration():
     try:
         ConversableAgent(name="agent", llm_config={"config_list": []})