Change default model from gpt-4o to gpt-4.1

[junichiro]

Overview

This pull request updates the default model used by ShellGPT from gpt-4o to gpt-4.1.

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Details

• Changed the value of DEFAULT_MODEL from gpt-4o to gpt-4.1 in the runtime configuration.
• This change will make ShellGPT use gpt-4.1 as the default model for new requests.

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Motivation

Recent updates from OpenAI have made the current gpt-4o model fully backward-compatible with gpt-4.1.
As a result, gpt-4.1 now offers both higher accuracy and better cost performance compared to gpt-4o.

Therefore, I would like to update the default model in this product to gpt-4.1 to ensure that users benefit from improved accuracy and reduced cost by default.

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Impact

• All new requests without an explicitly specified model will now use gpt-4.1.
• No major breaking changes are expected, but please let me know if any additional changes are needed.

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Thank you for reviewing this PR!