FEAT: Add missing OpenAI-compatible models (GPT-4.5, Claude models)

[SongChiYoung]

Why are these changes needed?

This PR adds missing model entries for OpenAI-compatible endpoints, including gpt-4.5-turbo, gpt-4.5-turbo-preview, and claude-3.5-sonnet. This improves coverage and avoids potential fallback or mismatch issues when initializing clients.

Checks

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[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.

Project coverage is 76.94%. Comparing base (7487687) to head (c02fee2).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...xt/src/autogen_ext/models/openai/_openai_client.py	20.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6120      +/-   ##
==========================================
- Coverage   76.96%   76.94%   -0.02%     
==========================================
  Files         192      192              
  Lines       13438    13445       +7     
==========================================
+ Hits        10342    10345       +3     
- Misses       3096     3100       +4     

Flag	Coverage Δ
unittests	76.94% <42.85%> (-0.02%)	⬇️

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