[db] Audit usage of "ON CONFLICT DO NOTHING RETURNING ..."

(here's some background that covers this use-case)

There are a handful of spots in the datastore where we:

• INSERT INTO a table,
• ON CONFLICT, DO NOTHING
• and have a RETURNING clause

Example: https://github.com/oxidecomputer/omicron/blob/3ae28c77cbd2355abeab07e358c5ccbeb035f641/nexus/src/db/datastore.rs#L880-L884

This is valid SQL, but it's a little suspect:

• If there are no conflicts (we're inserting the object with the UUID for the first time) we get the value that we intended to insert.
• However, if there are conflicts, this group of statements does not return any rows. Instead, it results in a NotFound error from the database.

This means that, especially in situations where we expect idempotency, this statement may return different results if invoked multiple times.

Start in nexus/src/db/datastore.rs around lines 880-884, then search the datastore for INSERT statements combining ON CONFLICT DO NOTHING with RETURNING. Review each use against the described conflict behavior; the audit is done when all affected usages have been identified and their inconsistent idempotent results are addressed.