Add NOT NULL (NOT VALID) constraint on internal_ids sharding key

What does this MR do and why?

This MR aims to accomplish two things because of its shear size exceeding the allotted runtime:

1. add a NOT NULL (NOT VALID) on the sharding key

2. prepare an async contraint validation

Addressing #451900 (closed) && #555251

There remain no more internal_ids rows without a sharding key on gitlab.com: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/41851/commands/128490

References

• #451900 (closed)
• &11670 (comment 2651132229)
• #555251

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