Add additional tests to capture edge cases and more error conditions

[umermansoor]

Why are these changes needed?

To enhance the test suite by adding new test cases to cover more edge cases.

Related issue number

Checks

• [x] I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• [x] I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [x] I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 11.60%. Comparing base (6279247) to head (f5ea3a9). Report is 54 commits behind head on main.

:exclamation: There is a different number of reports uploaded between BASE (6279247) and HEAD (f5ea3a9). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (6279247)	HEAD (f5ea3a9)
unittests	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3237       +/-   ##
===========================================
- Coverage   32.90%   11.60%   -21.30%     
===========================================
  Files          94       97        +3     
  Lines       10235    10812      +577     
  Branches     2193     2481      +288     
===========================================
- Hits         3368     1255     -2113     
- Misses       6580     9543     +2963     
+ Partials      287       14      -273     

Flag	Coverage Δ
unittests	11.60% <ø> (-21.30%)	:arrow_down:

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