Enforce issue URL requirement for tables with desired_sharding_key

What does this MR do and why?

This MR updates the sharding_key_spec to enforce stricter validation regarding sharding key definitions in db/docs/.

Previously, the spec did not enforce that tables defining a desired_sharding_key (indicating work-in-progress) must also provide a sharding_key_issue_url.

This MR changes the validation logic to:

1. Require a sharding_key_issue_url if a desired_sharding_key is present. This ensures all ongoing sharding work is tracked by a valid issue.
2. Forbid a sharding_key_issue_url if a final sharding_key is present (cleanup).
3. Introduce an allowlist (allowed_to_be_missing_sharding_key_issue_url) for existing tables that currently define a desired key but lack an issue URL. This prevents immediate spec failures while ensuring no new tables can be added without an issue link.

References

• Related to #580964 (closed)

Screenshots or screen recordings

N/A - Backend test change only.

Before	After

How to set up and validate locally

1. Verify the specs pass with the current allowlist:

   bin/rspec spec/lib/gitlab/database/sharding_key_spec.rb

2. Verify the new validation (Failure scenario):

   • Pick a random table definition in db/docs/ (e.g., db/docs/users.yml).
   • Add desired_sharding_key: project_id but do not add a sharding_key_issue_url.
   • Run the spec: bin/rspec spec/lib/gitlab/database/sharding_key_spec.rb
   • Expected Result: The test should fail asking for a URL.

3. Verify the allowlist enforcement (Cleanup scenario):

   • Pick a table currently in the new allowlist (e.g., spam_logs).
   • Edit db/docs/spam_logs.yml and add a valid sharding_key_issue_url.
   • Run the spec: bin/rspec spec/lib/gitlab/database/sharding_key_spec.rb
   • Expected Result: The test should fail stating the table must be removed from the exemption list.

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