Feat/gpt5 support

[tejas-dharani]

Why are these changes needed?

This PR introduces GPT-5 support across the Autogen core and extensions, enabling developers to build agents and tools that fully utilize GPT-5’s advanced reasoning and tool-use capabilities while preserving backward compatibility with existing Autogen behaviors. Updated OpenAI package to support GPT 5.

These changes unlock GPT-5’s reasoning abilities, custom tool integration, and Responses API features for more capable, context-aware agents.

Related issue number

Closes #6916

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

❌ Patch coverage is 86.51685% with 60 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.05%. Comparing base (a666c65) to head (2cbb7c1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...src/autogen_ext/models/openai/_responses_client.py	82.42%	42 Missing ⚠️
...xt/src/autogen_ext/models/openai/_openai_client.py	85.71%	14 Missing ⚠️
...gen-ext/src/autogen_ext/tools/graphrag/__init__.py	85.71%	2 Missing ⚠️
...kages/autogen-core/src/autogen_core/tools/_base.py	98.36%	1 Missing ⚠️
...n-ext/src/autogen_ext/models/openai/_model_info.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6917      +/-   ##
==========================================
+ Coverage   80.89%   81.05%   +0.15%     
==========================================
  Files         237      239       +2     
  Lines       18349    18774     +425     
==========================================
+ Hits        14844    15217     +373     
- Misses       3505     3557      +52     

Flag	Coverage Δ
unittests	81.05% <86.51%> (+0.15%)	⬆️

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[ekzhu]

It is a lot of changes, can we separate them into different PRs? Have 1 PR for the model info updates and another PR for the response client.

[Kayaba-Attribution]

what is the status of this integration? I am looking forward to use gpt-5 on my project. Any other PRs that have done the changes?