feat: Implement OpenAI Responses API

[iGore]

This commit introduces the implementation of the OpenAI Responses API, allowing users to leverage tools like web search.

• Adds response.go with the CreateResponseRequest and CreateResponseResponse structs and the CreateResponse client method.
• Includes response_test.go with a unit test to verify the new functionality.

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[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.51%. Comparing base (5d7a276) to head (8ba2d4a).

Files with missing lines	Patch %	Lines
response.go	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1089      +/-   ##
==========================================
- Coverage   99.59%   99.51%   -0.08%     
==========================================
  Files          34       35       +1     
  Lines        2209     1852     -357     
==========================================
- Hits         2200     1843     -357     
+ Misses          6        5       -1     
- Partials        3        4       +1     

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