feat: add GPT-4o mini to GitHub Copilot provider

[I3eka]

Summary

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

Changes

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

Motivation

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

[Copilot]

Pull request overview

Adds gpt-4o-mini as a selectable model under the GitHub Copilot (gh) provider in the shared provider model registry.

Changes:

• Added gpt-4o-mini to the gh provider model list in PROVIDER_MODELS.

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[Copilot]

Adding gpt-4o-mini as the first entry in the gh model list changes the provider’s default model because getDefaultModel() returns models?.[0]?.id. If the default for GitHub Copilot is intended to remain gpt-4.1, append this model later in the list or introduce an explicit per-provider default selection instead of relying on array order.

Suggested change

	{ id: "gpt-4o-mini", name: "GPT-4o mini" },
	{ id: "gpt-4.1", name: "GPT-4.1" },
	{ id: "gpt-4.1", name: "GPT-4.1" },
	{ id: "gpt-4o-mini", name: "GPT-4o mini" },

[Copilot]

Model display name casing is inconsistent: this entry uses "GPT-4o mini" while other "Mini" variants in this list (e.g., "GPT-5 Mini") and the openai provider list use "GPT-4o Mini". Consider standardizing the casing to avoid duplicate-looking entries or inconsistent UI labels.

Suggested change

	{ id: "gpt-4o-mini", name: "GPT-4o mini" },
	{ id: "gpt-4o-mini", name: "GPT-4o Mini" },