prompts: Improve inline assist prompt to reduce garbage from smaller models (#38278)

Closes #24412 and #19471

I tested both insertion and replacing with o3-mini and it failed with
the current prompt. With the updated prompt it does no longer return
`<document><rewrite_this>` or `{{REWRITTEN_CODE}}`

I have ensured the LLM Worker works with these prompt changes.

Release Notes:

- Improved prompting for the inline assistant
d1543f75b6Andreas Johansson committed on 10/1/2025, 9:07:57 AM· committed by GitHubparentfc0b249
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