Move behavior tests off private shapes

[AustinKelsay]

Parent

#130

What to build

Replace private implementation casts in the Relay, Nostr, NIP-46, and NIP-47 test clusters with behavior observed through public APIs or narrowly owned testing-entrypoint seams.

Acceptance criteria

• The shared private-shape test adapters for Nostr and Relay are removed.
• Relay behavior tests no longer cast instances to private fields or methods.
• Targeted NIP-46 and NIP-47 tests no longer reach through broad private-shape casts.
• Any remaining test-only control is explicit, narrow, and owned by the testing entrypoint.
• Behavioral coverage and failure sensitivity remain intact.

Blocked by

• Shorten the default test feedback loop #137

Delivery

This is cleanup slice 8 of 9. Deliver it on its own branch and non-draft pull request into staging, then merge it before starting the next slice.

[AustinKelsay]

Completed and merged into staging via #147 (merge commit 3c1e905). All hosted Node 16/18/20 and Bun checks passed; hosted CodeRabbit findings were fixed or explicitly withdrawn.