Update agent_persist_mock to patch _exists method

[nassermohamedit0]

This PR updates persist_agent_mock to patch the _exists method that was added to the agent_persist_mixin.

This PR also updates the patch function _add to only encode the value argument if it is not of type bytes.
If value is of type bytes then we have str(value).encode() != value.

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 62.54%. Comparing base (2a0f88e) to head (6819446).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ostorlab/testing/agent.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #868      +/-   ##
==========================================
+ Coverage   62.52%   62.54%   +0.02%     
==========================================
  Files         342      342              
  Lines       14545    14553       +8     
==========================================
+ Hits         9094     9102       +8     
  Misses       5451     5451              

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[ostorlab-ai-pr-review]

Code Review Summary

Total Issues Found: 1
Critical Issues: 1
Suggestions: 1

Key Findings:
The code review identified a critical issue in src/ostorlab/testing/agent.py related to inconsistent data encoding in the storage mechanism. Specifically, line 102 bypasses the encoding applied in line 100, potentially leading to inconsistencies when storing bytes values. It was suggested to either remove line 102 or explicitly encode bytes values before assignment. Also, it was suggested to add a comment explaining why the encoding is necessary. Overall code quality is impacted due to this inconsistency.