Handle Invalid LFS Batch Objects

What does this MR do and why?

Problem

LFS batch requests with objects sent as a hash caused a 500 because ActionController::Parameters does not support to_a. These requests should return the existing 404 response for invalid object collections.

Solution

• Convert objects only when Array.try_convert accepts the value.
• Treat non-array object input as empty.
• Add request specs for hash and numeric-keyed hash payloads.

References

• Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/4160363

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.