test: Refactor Payjoin integration tests & helpers

[ValeraFinebits]

Adds the missing negative integration test for Invalid invoices and cleans up the related payjoin failure coverage.

Included in this change

• add PayjoinRequestFailsWhenInvoiceBecomesInvalid()
• group the three related negative tests at the bottom of PayjoinPluginIntegrationTests
• make the assertions consistent across Paid, Expired, and Invalid
• use a race-safe helper for receiver session shutdown (close requested or already removed)

Notes
These tests currently assert the observable behavior as it exists today: the external payjoin payer times out after the invoice becomes non-payable.
Each test keeps a TODO to tighten this to an explicit fast rejection once the payjoin library supports clean in-flight session rejection.

[chavic]

I’m okay with this as a consistency refactor around the plugin’s current behavior. My main concern was making sure we weren’t accidentally turning timeout into the intended contract for already-paid, expired, and invalid invoice cases. Since those paths are still explicitly marked as TODO pending clean in-flight session closure support in the payjoin library, this makes sense to me as an intermediate step. Approving with the expectation that we’ll tighten these cases later once the underlying receiver rejection behavior is available.