[codex] Use wpdb insert identity field

[alexstandiford]

Summary

• Resolve auto-increment insert identities from the table's declared identity field instead of hardcoding id.
• Treat a missing wpdb->insert_id as a datastore failure after a successful insert call.
• Add unit coverage for provided identities, auto-increment identities, and unresolved identities.

Why

WordPress' insert path calls $wpdb->insert() and then uses $wpdb->insert_id from the same database connection. This keeps identity resolution in the write path instead of requiring callers to infer or re-query the record.

Validation

• vendor/bin/phpunit --filter CanQueryWordPressDatabaseTest --testdox
• vendor/bin/phpunit --testdox

[alexstandiford]

Closing this because it only validates the wpdb insert ID in the WordPress adapter. It does not address the actual create-path issue, which is that PHPNomad db immediately rereads after insert instead of following WordPress' insert-then-use-identity pattern.