chore: Add respx dependency and update unit tests to use mocks

[chyroc]

• Added respx dependency for HTTPX mocking
• Updated all unit tests to execute based on mocks
• Removed dependency on environment variables for unit tests and remove from ci.yaml configuration

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.85%. Comparing base (19f9840) to head (eb99ac3).
Report is 4 commits behind head on main.

@@             Coverage Diff             @@
##             main      #47       +/-   ##
===========================================
+ Coverage   71.86%   85.85%   +13.98%     
===========================================
  Files          30       30               
  Lines        1134     1258      +124     
===========================================
+ Hits          815     1080      +265     
+ Misses        319      178      -141     

Flag	Coverage Δ
	85.85% <100.00%> (+13.98%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
cozepy/auth/__init__.py	82.91% <100.00%> (+19.09%)	⬆️
cozepy/bots/__init__.py	100.00% <100.00%> (+14.28%)	⬆️
cozepy/chat/__init__.py	69.46% <100.00%> (-15.27%)	⬇️
cozepy/files/__init__.py	100.00% <100.00%> (+42.85%)	⬆️
cozepy/knowledge/documents/__init__.py	97.02% <100.00%> (+22.77%)	⬆️
cozepy/request.py	80.26% <100.00%> (+3.94%)	⬆️
tests/config.py	100.00% <100.00%> (+33.33%)	⬆️
tests/test_auth.py	100.00% <100.00%> (ø)
tests/test_bot.py	100.00% <100.00%> (ø)
tests/test_conversation_message.py	100.00% <100.00%> (+73.91%)	⬆️
... and 3 more

... and 8 files with indirect coverage changes