Ignore bad values sent to `commit_message` in suggestion API

What does this MR do?

Adds a coerce function to the commit_message parameter in the suggestion API that will reject any strange values

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

Related to #300139 (closed)

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