change create_ methods into upsert_

[anhqle]

Summary

This PR changes the create_somenode methods into upsert_somenode, which create if a node doesn't exist and update if it does exist.

This is useful because users just want to "make the nodes this way" without having to first check for existence. An example use case is writing a notebook with a bunch of upsert, which would be reproducible and idempotent.

The commit so far only changes create_source into upsert_source. If the approach makes sense I'll make the same changes to other nodes types & and add tests.

Test Plan

• [ ] PR has an associated issue: #
• [ ] make check passes
• [ ] make test shows 100% unit test coverage

Deployment Plan

[anhqle]

• #996 👈
• main

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[anhqle]

This functionality is added in #1001 instead