Ensure suggestions sharding key is set at the application level

What does this MR do and why?

This is part of the work needed for sharding the table suggestions (Re #550696):

Tasks	MR
Add sharding key column namespace_id Add a trigger to populate this column on insert and update Add NOT VALID NOT NULL constraint Create an index Add NOT VALID foreign key	✅ !207836 (merged)
Ensure namespace_id is set at the application level	🚧 !210092 (merged) 👈 we are here
Backfill namespace_id for existing records	⏳ !209953
Finalise backfilling migration Validate NOT NULL constraint and foreign key Update model validation to include all actions	⏳ TBD

Summary of changes

• Add a before_validation callback to set namespace_id on create
• Validate presence of namespace on create (unless the record is being imported)
• Update Suggestions::CreateService to set namespace_id when creating a suggestion
• Update specs to ensure the namespace_id value is being set for these records (including during a project import)

To note: In Suggestion#ensure_namespace_id, we have to check for note.project.project_namespace_id because note.namespace_id (that table sharding key) might not be present yet. We do know that these kinds of notes belong to a project, so if the note is present, we can fetch the namespace via its project.

References

Related to #572849

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