fix:Introduce _is_python_code check into _extract_code

[adamingas]

• Add tests to cover more cases for _extract_code and _is_python_code

• Addresses 'No code found in the response' error - retry code gen with chatgpt #1128
• Tests added and passed if fixing a bug or adding a new feature.
• All code checks passed.

Motivation:
Using a dataframe with a large number of columns, the LLM()._execute_code method would fail when asking how many rows there are in the dataframe. The failure would be because the LLM wasn't using triple back ticks to surround the code.

If the LLM produces correct python code, but for some reason does not
preface it with ```, the LLM()._extract_code method fails with
NoCodeFoundError.

In this change we first check if the separator is included in the response and we take only the part that is in-between. Then we clean the code from python/py statements.
Then we check if it's valid code. This check should pass otherwise executing the code will fail, and it's better to raise an error earlier than later.

With this change, if the LLM produced perfect code without the ``` separator, it would pass and return the code, whereas previously it would fail.

[tushar-31093]

Guys, this is a genuine concern, could you confirm if this fix might solve this problem?

[adamingas]

@gventuri Can someone take a look at the PR? Happy to change stuff if need be.

[gventuri]

@adamingas merged, thanks a lot for the PR :D