Correct and clean up xray_report payload schema & factory

What does this MR do and why?

In #479185 (closed), we updated the Projects::XrayReport model's payload JSON schema. There was a small typo in the issue which said to use file_path, but it was supposed to be the plural file_paths. This is because when we save the report in Ai::RepositoryXray::ScanDependenciesService, we merge multiple payloads and combine the file paths into a string array.

This MR:

• Updates the Xray payload JSON schema with file_paths and defines it as a string array.
  • NOTE: This does not affect any existing code/behaviour because:
    • (a) we currently don't make use of the file_paths value anywhere, and
    • (b) the JSON schema is not and has never been restrictive so far (i.e. no required fields), so it won't throw validation errors.
• Updates the xray_reports factory and related specs to more accurately reflect actual values saved to the database.

References

• Related to issue: Update `XrayReport` model JSON schema & remove ... (#479185 - closed)

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