fix: use system prompts for default endpoint type instances in next edit

[uinstinct]

Description

For default endpoint type instances in next edit, send both the user and system prompts.

closes #8590
resolves CON-4802

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Summary by cubic

Next Edit now sends both the system and user prompts to default endpoint type models; fine-tuned models still receive only the user prompt. Fixes missing system context to improve responses; resolves CON-4802 and closes #8590.

^{Written for commit d62bb21. Summary will update automatically on new commits.}

[cubic-dev-ai]

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