fix: leave worker registry unchanged on alias collision

[flcrom]

Fixes #1433.

Workers.add currently writes the primary kind and earlier aliases before checking later aliases. When a later alias is already registered, AddWorkerSafely returns an error but leaves those new kinds in the registry, preventing a caller from registering replacement workers.

Collect and validate every kind before writing to the map. Keep the existing collision error text and rejection of duplicate kinds within a registration. Add a regression for a late alias collision that checks the registry size and absence of the candidate primary kind and earlier alias.

Validation:

• The regression fails on the original implementation and passes with the patch, including 30 race-detector repetitions.
• Public-API recovery and edge checks cover primary/alias collisions, duplicate aliases, preserved occupied workers and valid registrations.
• Full make test, PostgreSQL and SQLite make test/race, and make lint pass across all nine workspace modules. Lint used v2.13.1 with concurrency=1 for local memory limits.

Local testing used Go 1.26.6 and PostgreSQL 14.24 with UTF8. This does not claim every upstream CI OS/Go/database combination has passed. No registration-after-start behavior is changed.

[brandur]

@flcrom Do you want to add yourself to the River CLA https://github.com/riverqueue/rivercla? And add a changelog entry for this as well please.

[flcrom]

CLA is already signed: I'm in signees.csv in riverqueue/rivercla (riverqueue/rivercla#37). Changelog entry added in 3f38b47.

[brandur]

@flcrom Ah perfect. Thanks again!