feat: add GPT-4 to GitHub Copilot provider

[I3eka]

Summary

Added GPT-4 to the list of supported models for the GitHub Copilot (gh) provider.

Changes

• Updated PROVIDER_MODELS constant.
• Added { id: "gpt-4", name: "GPT-4" }, to the gh array.

Motivation

Enables users to select GPT-4 when using the GitHub Copilot provider key.

[Copilot]

Pull request overview

Adds GPT-4 as a selectable model for the GitHub Copilot (gh) provider by extending the provider model allowlist used across open-sse.

Changes:

• Added { id: "gpt-4", name: "GPT-4" } to the gh provider’s model list.
• Updated the PROVIDER_MODELS constant accordingly.

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[Copilot]

Adding gpt-4 at the top of the gh model list changes the default model for the GitHub Copilot provider because getDefaultModel() returns the first entry in PROVIDER_MODELS[alias]. If the intent is only to add support (not change defaults), place gpt-4 after the current default (or explicitly keep the desired default first) and/or document the default-model ordering expectation.

Suggested change

	gh: [ // GitHub Copilot - OpenAI models
	{ id: "gpt-4", name: "GPT-4" },
	{ id: "gpt-4.1", name: "GPT-4.1" },
	gh: [ // GitHub Copilot - OpenAI models (first entry is the default model)
	{ id: "gpt-4.1", name: "GPT-4.1" },
	{ id: "gpt-4", name: "GPT-4" },