fix: deepseek doesn't support 'https://api.deepseek.com/completions' …

[imcheeseberg]

Deepseek doesn't support 'https://api.deepseek.com/completions' URL currently. When user selecting code in editor, and then use 'CMD +I' command to tell model to modify codes, 400 error happens.

Description

when providerName is deepseek , supportsCompletions() should return false.

Checklist

• [x] The base branch of this PR is dev, rather than main
• [x] The relevant docs, if any, have been updated or created

Screenshots

Before

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Testing

[ For new or modified features, provide step-by-step testing instructions to validate the intended behavior of the change. ]