Add end-to-end coverage for the registry workflows that have none

[lmcorbalan]

User story / Problem statement

Currently, the registry's end-to-end suite covers the offer transfer path, the direct transfer path, instrument metadata plus the faucet, and the sandbox boot checks. Several workflows the Daml implements and the service serves have no live coverage at all, so the only thing standing between a Daml change and a broken route is a manual check somebody remembers to run. The unit suite cannot close this gap: it runs against a stub ledger and is blind to anything on the Daml side by construction.

Expected outcome

Every workflow the service exposes is exercised at least once against a live participant, so a Daml change that strands a route fails a suite instead of surfacing during a manual bring-up.

Acceptance criteria

• The allocation path has end-to-end coverage: the allocation factory route, and the execute-transfer, withdraw and cancel choice contexts.
• The transfer instruction reject choice context has end-to-end coverage.
• The remaining gaps are enumerated when this is picked up, and either covered or written down as deliberate exclusions.

Technical notes

Deliberately brief: this is a placeholder to get the work on the board. The full inventory of what is and is not covered belongs in the analysis when someone starts on it.

The allocation path was confirmed working by hand against a local sandbox, so this is about locking that in, not about fixing a known break.