Fix: [Bug] NameError: name 'p' is not defined #15581

[LucasTonetto]

Modification of the markdown parser method to remove "python" from the markdown before processing it, if it exists

Description

Fixes #15581
When VertexAI is used, the return of the markdown text passed to the parse_code_markdown function uses language markup. For example ```python df.columns```. The previous code only considered removing the backticks. Treatment has been implemented to remove the language markup when it is present. Additionally, for string type returns, the snippet parse_code_markdown(output, only_last=True)[0] only returned the first letter of the string.

Error example:

> Pandas Instructions:
\```
\```python
df.groupby('filial')['forma_pagamento'].agg(lambda x: x.value_counts().idxmax())
\```
\```
Traceback (most recent call last):
  File "C:\...\llama_index\.venv\Lib\site-packages\llama_index\experimental\query_engine\pandas\output_parser.py", line 63, in default_output_processor
    output_str = str(safe_eval(module_end_str, global_vars, local_vars))
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\...\llama_index\.venv\Lib\site-packages\llama_index\experimental\exec_utils.py", line 159, in safe_eval
    return eval(__source, _get_restricted_globals(__globals), __locals)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<string>", line 1, in <module>
NameError: name 'p' is not defined

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

• I added new unit tests to cover this change
• I believe this change is already covered by existing unit tests

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks.
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I ran make format; make lint to appease the lint gods

[LucasTonetto]

@logan-markewich Apologies for the oversight. :(
This is my first pull request for a major project, and I was a bit anxious. The linting issue has been fixed. Please review the PR again.