Ensure SecretStr is cast to str on load for model clients

[victordibia]

Why are these changes needed?

Currently we have SecretStr type for model clients to promote security best practices.

• when we dump_component, keys are serialized as SecreteStr ..
• when we load_component ... SecreteStr type is passed to the client in the api_key field. This i causes the type problems as the clients expect a string type.

This PR updates the from_config method for model clients to ensure we get the value from SecretStr.

Related issue number

Closes #5944

Checks

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

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 75.76%. Comparing base (296de52) to head (e2a1e5d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...xt/src/autogen_ext/models/openai/_openai_client.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5947      +/-   ##
==========================================
- Coverage   75.77%   75.76%   -0.01%     
==========================================
  Files         191      191              
  Lines       13100    13107       +7     
==========================================
+ Hits         9927     9931       +4     
- Misses       3173     3176       +3     

Flag	Coverage Δ
unittests	75.76% <75.00%> (-0.01%)	⬇️

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