Deprecate and remove remaining bucket key-based GraphQL/Liquid surface

[nbudin]

Fixes #11896

Summary

Most of the fields this issue lists were already deprecation_reason-marked (the four Signup-family types' bucketKey/requestedBucketKey, and all six mutations' bucket_key/requested_bucket_key arguments) — confirmed by reading the code before starting. So the actual work here is narrower than the issue's background section:

• Removed BucketKeyMappingInputType#from_key entirely (not just deprecated it) — a removed bucket is always a persisted row with a real id already, unlike to_key's bucket, which may be newly created in the same edit and have no id yet. This required converting BucketKeyRemappingModal.tsx/useBucketKeyRemapping.tsx, which previously always sent from_key, to submit from_bucket_id instead — the removed bucket's real id was already available via RegistrationPolicyBucket#as_json (from Give the registration policy editor real bucket ids (phase 1 of dropping RegistrationPolicyBucket#key) #11895), just not yet plumbed through these components' TypeScript types.
• Deprecated RegistrationPolicyBucketType#key — the one field the issue flagged as not yet deprecated. This surfaces new @graphql-eslint/no-deprecated warnings on the many places that still legitimately read key for editing/remapping (confirmed non-blocking — 0 new errors from a full yarn eslint app/javascript sweep). Addressing those reads is out of scope for this issue; that's 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.
• Removed RegistrationPolicy::BucketDrop#key's delegate (confirmed zero real-world Liquid usage per the issue's own research).
• Deleted the dead RegistrationPolicyInputType/RegistrationPolicyBucketInputType — confirmed referenced by nothing, not even present in the generated schema before this change.

Deliberately not doing: removing the already-deprecated bucketKey/requestedBucketKey fields or mutation arguments. The issue calls for that to wait on a deprecation window passing — a timing/process decision, not something to execute automatically here.

Test plan

• bin/rails graphql:update (confirmed the schema diff is exactly the two expected changes: from_key removed, key deprecated — no stray diffs)
• yarn run tsc --noEmit
• yarn vitest run (187 passed, 1 pre-existing skip)
• bin/rails test (1169 passed, 1 pre-existing skip)
• bundle exec rubocop / yarn eslint on all changed files (0 errors)
• Full yarn eslint app/javascript sweep to confirm the new key deprecation produces only warnings, no errors

🤖 Generated with Claude Code

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
app/graphql/intercode_schema.rb	🟠 55.26%	🟠 61.84%	🟢 6.58%
app/javascript/BuiltInFormControls/AddFileModal.tsx	🔴 0%	🔴 33.33%	🟢 33.33%
app/javascript/BuiltInFormControls/LiquidInput.tsx	🟠 57.5%	🟠 60%	🟢 2.5%
app/models/deprecated_graph_ql_usage.rb	🔴 0%	🟢 100%	🟢 100%
Overall Coverage	🟢 56%	🟢 56.04%	🟢 0.04%

Minimum allowed coverage is 0%, this run produced 56.04%