add GPT-5 model constants and reasoning validation

[behzadsp]

Add GPT-5 model constants and reasoning validation

This PR adds support for OpenAI's newly released GPT-5 model series and extends the existing reasoning model validation to include GPT-5 models.

Model Constants Added:

• GPT5 - Main GPT-5 model (gpt-5)
• GPT5Mini - Smaller variant (gpt-5-mini)
• GPT5Nano - Fastest variant (gpt-5-nano)
• GPT5ChatLatest - Latest chat-optimized variant (gpt-5-chat-latest)

Also extended ReasoningValidator to include GPT-5 series models as GPT-5 models now validate parameter restrictions like O1/O3/O4 series:

• logprobs not supported
• temperature, top_p, n fixed at 1
• presence_penalty, frequency_penalty fixed at 0

Testing

• ✅ All existing tests pass
• ✅ New TestGPT5ModelsChatCompletionsBetaLimitations validates parameter restrictions
• ✅ Updated TestCompletionWithGPT5Models for completions endpoint restrictions

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.59%. Comparing base (c4273cb) to head (eb991b1).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1062   +/-   ##
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  Coverage   99.59%   99.59%           
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  Files          34       34           
  Lines        2205     2206    +1     
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+ Hits         2196     2197    +1     
  Misses          6        6           
  Partials        3        3           

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[Copilot]

Pull Request Overview

This PR adds support for OpenAI's newly released GPT-5 model series by introducing model constants and extending the existing reasoning model validation framework to include GPT-5 models.

• Added four new GPT-5 model constants (GPT5, GPT5Mini, GPT5Nano, GPT5ChatLatest)
• Extended ReasoningValidator to apply parameter restrictions to GPT-5 models (same limitations as O1/O3/O4 series)
• Updated completion endpoint restrictions to disable GPT-5 models from the completions API

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
completion.go	Adds GPT-5 model constants and disables them for completion endpoint
reasoning_validator.go	Extends validation logic to include GPT-5 series models with parameter restrictions
completion_test.go	Tests that GPT-5 models are properly rejected by completion endpoint
chat_test.go	Comprehensive test coverage for GPT-5 model parameter restrictions in chat completions

[sashabaranov]

Thank you for the PR!