fix: improve error message for missing fix function in suggestions

[jaymarvelz]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[x] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

When a rule's suggestion is missing a fix function, the error message displayed [object Object] instead of showing the actual suggestion object contents.

What changes did you make? (Give an overview)

Updated the error message in validateSuggestions() to use JSON.stringify(suggestion, null, 2) instead of string interpolation.

Before:

TypeError: context.report() called with a suggest option without a fix function. See: [object Object]

After:

TypeError: context.report() called with a suggest option without a fix function. See: {
  "desc": "Some suggestion description",
}

Is there anything you'd like reviewers to focus on?

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[DMartens]

Thanks for spotting this, changes LGTM.
Is there any reason this PR is marked as a draft?

[mdjermanovic]

LGTM, thanks! Just waiting for the current release to conclude.

[fasttime]

LGTM, thanks!