Description

fim_prefix tag was being added in the prefix of autocomplete requests to mercury coder which were unnecessary. This PR fixes that.

resolves CON-4805

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• [] I've read the contributing guide
• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created

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Summary by cubic

Removed the fim_prefix marker from Mercury coder autocomplete prefixes to stop sending an unnecessary tag and fix prompt formatting. Aligns the FIM template with model expectations and resolves CON-4805.

^{Written for commit df363f5. Summary will update automatically on new commits.}