Validate llm_config passed to ConversableAgent (issue #1522) (#1654)

* Validate llm_config passed to ConversableAgent

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

Add the following validation and `raise ValueError` if:

 - The `llm_config` is `None`.
 - The `llm_config` is valid, but `config_list` is missing or lacks elements.
 - The `config_list` is valid, but no `model` is specified.

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

* Validate llm_config passed to ConversableAgent

Based on #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.

* Validate llm_config passed to ConversableAgent

Based on #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.

* Validate llm_config passed to ConversableAgent

Based on #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.

* Validate llm_config passed to ConversableAgent

Based on #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.

* Fix the test_web_surfer issue

For anyone reading this: you need to `pip install markdownify` for the
`import WebSurferAgent` to succeed. That is needed to run the
`test_web_surfer.py` locally.

Test logic needs `llm_config` that is not `None` and that is not
`False`.

Let us pray that this works as part of GitHub actions ...

* One more fix for llm_config validation contract