Deprecate and remove the remaining bucket key-based GraphQL/Liquid surface (phase 2 of dropping RegistrationPolicyBucket#key)

[nbudin]

Background

Once #11895 lands, nothing in the frontend still needs to read or write RegistrationPolicyBucket#key (or the deprecated signup-side bucketKey/requestedBucketKey fields) as identity -- everything goes through id instead. This issue is the GraphQL-schema half: deprecate, then remove, the key-based surface that's no longer needed, so #11897 and #11898 can proceed without an external API still depending on it.

Concretely, the following are still live in the schema today:

• Types::BucketKeyMappingInputType (app/graphql/types/bucket_key_mapping_input_type.rb): from_key/to_key arguments, both already deprecation_reason-marked in favor of from_bucket_id/to_bucket_id.
• Deprecated bucketKey/requestedBucketKey fields on Types::SignupType, Types::SignupRequestType, Types::SignupRankedChoiceType, Types::SignupChangeType.
• Deprecated bucket_key/requested_bucket_key mutation arguments on Mutations::CreateMySignup, Mutations::CreateUserSignup, Mutations::CreateSignupRequest, Mutations::CreateSignupRankedChoice, Mutations::ForceConfirmSignup, Mutations::UpdateSignupBucket.
• Types::RegistrationPolicyBucketType#key (app/graphql/types/registration_policy_bucket_type.rb) -- the bucket's own read field, not currently marked deprecated at all. Convert remaining registration policy bucket key dependencies (SignupBucketFinder, EventChangeRegistrationPolicyService) to id #11892's research found no Liquid template usage of the equivalent RegistrationPolicy::BucketDrop#key field; this issue is about the GraphQL field specifically, which Give the registration policy editor real bucket ids (phase 1 of dropping RegistrationPolicyBucket#key) #11895's frontend conversion makes unused by this codebase's own frontend (though it's a public field on a public API -- see Tradeoffs).
• RegistrationPolicy::BucketDrop#key (app/liquid_drops/registration_policy/bucket_drop.rb) -- confirmed zero real-world usage across ~170 conventions' Page/CmsPartial/CmsLayout/NotificationTemplate/Product content in a full-text search.

Also found along the way, and unrelated to any of this except by proximity: Types::RegistrationPolicyInputType/Types::RegistrationPolicyBucketInputType (with a required: true key argument) are referenced by no mutation argument and no frontend query at all -- dead code, safe to delete independently of everything else in this plan.

Proposed Change

1. Remove from_key from BucketKeyMappingInputType -- fully covered by from_bucket_id, since a removed bucket is always a persisted row with a real id already (unlike a bucket being newly created in the same edit, which to_key still needs to cover).
2. Deprecate, then (after your normal deprecation window) remove: the bucketKey/requestedBucketKey fields on the four Signup-family GraphQL types, the bucket_key/requested_bucket_key mutation arguments on the six mutations listed above, and RegistrationPolicyBucketType#key.
3. Remove RegistrationPolicy::BucketDrop#key's delegate.
4. Delete the dead RegistrationPolicyInputType/RegistrationPolicyBucketInputType types.
5. Run bin/rails graphql:update and commit the regenerated schema/generated frontend files, per this repo's CLAUDE.md.

Benefits

• Removes every remaining external-facing dependency on bucket key-as-identity, so the app-layer and database cleanups (Retire RegistrationPolicyBucket#key as an identity mechanism in the app layer (phase 3 of dropping RegistrationPolicyBucket#key) #11897, Drop the registration_policy_buckets.key column (phase 4 of dropping RegistrationPolicyBucket#key) #11898) don't have to preserve a public API around it.
• Deletes a chunk of confirmed-dead GraphQL types.

Tradeoffs

• Steps 1-3 are breaking API changes to a schema that (per this repo's model) may have external consumers beyond this codebase's own frontend. Needs whatever deprecation-window/versioning process this project normally applies before removing a deprecated field -- this issue is scoped to include both the deprecation and the eventual removal, but the removal step shouldn't happen until that window has passed.
• RegistrationPolicyBucketType#key isn't currently marked deprecation_reason at all, so removing it has a longer runway to plan for than the fields that are already deprecated.

References

• Parent: Convert bucket_key/requested_bucket_key to real foreign keys #11868
• Depends on: Give the registration policy editor real bucket ids (phase 1 of dropping RegistrationPolicyBucket#key) #11895 (must land first, so nothing in this codebase's own frontend still reads these fields)
• Part of a 4-phase plan to eventually drop registration_policy_buckets.key; see Give the registration policy editor real bucket ids (phase 1 of dropping RegistrationPolicyBucket#key) #11895, Retire RegistrationPolicyBucket#key as an identity mechanism in the app layer (phase 3 of dropping RegistrationPolicyBucket#key) #11897, Drop the registration_policy_buckets.key column (phase 4 of dropping RegistrationPolicyBucket#key) #11898

---

Issue drafted by Claude