Convert remaining registration policy bucket key dependencies to id

[nbudin]

Fixes #11892

Summary

Follow-up to #11870/#11891, which handled bucket key/id correlation only for editing an existing registration policy. This converts the remaining internal identity uses of RegistrationPolicyBucket#key — in SignupBucketFinder, RegistrationPolicyBucket's occupancy checks, and EventChangeRegistrationPolicyService's simulation/removed-bucket logic — to use id for persisted buckets, or object identity for not-yet-persisted candidate buckets during registration policy simulation (a bucket being added in the same edit has no real id until the edit is persisted, so key is the only identity that predates the object entirely disappearing — object identity replaces it everywhere it's actually needed).

• RegistrationPolicyBucket#occupies_bucket_as_signup?/#signup_definitely_occupies_slot_in_bucket?: single id-or-object-identity comparison, key-based fallback removed.
• SignupBucketFinder/FakeSignup: track bucket objects (bucket/requested_bucket) instead of key strings.
• EventChangeRegistrationPolicyService: bucket_key_mappings resolves to id (bucket_id_mappings) up front instead of round-tripping through a removed bucket's key at every use site; the simulation bridges a real signup's persisted bucket to its detached candidate counterpart via one clearly-commented key lookup (structurally required — see comments), instead of using key everywhere.
• Removed RegistrationPolicy#bucket_with_key and the bucket_key/requested_bucket_key readers on Signup/SignupRequest/SignupRankedChoice, since SignupBucketFinder was meant to be their only caller. Turned out there were a handful of other legitimate callers (deprecated GraphQL bucketKey/requestedBucketKey mutation arguments, and import_convention_data_service.rb) — those got an inlined equivalent lookup rather than resurrecting the shared method.
• sync_buckets_from_hash!'s key-based fallback (from Correlate registration policy buckets by id instead of key when editing/simulating policy changes #11870) is untouched, as planned — dropping the key column is tracked separately.

No GraphQL/frontend changes

update_event's bucket_key_mappings argument, BucketKeyMappingInputType, and the frontend bucket remapping UI are unchanged. The registration policy editor never has real bucket ids for candidate buckets (confirmed via RegistrationPolicy#as_json/RegistrationPolicyBucket#as_json, which deliberately omit id), so to_key must stay key-based — it can name a bucket being created in the very same edit.

Test plan

• bin/rubocop clean on all changed files
• Full affected model/service/graphql test suites pass (274 tests, 0 failures)
• Reviewed diff manually for the trickier identity-comparison edge cases (detached candidate buckets, FakeSignup wiring, RegistrationPolicyBucketTest#signup_definitely_occupies_slot_in_bucket?)

🤖 Generated with Claude Code

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
app/graphql/mutations/create_my_signup.rb	🟢 95.65%	🟢 96%	🟢 0.35%
app/graphql/mutations/create_signup_ranked_choice.rb	🟠 68.42%	🟠 62.5%	🔴 -5.92%
app/graphql/mutations/create_signup_request.rb	🟠 62.5%	🟠 57.14%	🔴 -5.36%
app/graphql/mutations/create_user_signup.rb	🟠 55.56%	🟠 51.72%	🔴 -3.84%
app/models/registration_policy.rb	🟢 98.68%	🟢 98.65%	🔴 -0.03%
app/models/signup.rb	🟢 90.28%	🟢 89.71%	🔴 -0.57%
app/models/signup_request.rb	🟢 95.83%	🟢 95.45%	🔴 -0.38%
app/services/event_change_registration_policy_service.rb	🟢 91.96%	🟢 92.66%	🟢 0.7%
app/services/import_convention_data_service.rb	🟢 97.11%	🟢 97.13%	🟢 0.02%
app/services/signup_bucket_finder.rb	🟢 81.69%	🟢 81.43%	🔴 -0.26%
Overall Coverage	🟢 56.02%	🟢 56.03%	🟢 0.01%

Minimum allowed coverage is 0%, this run produced 56.03%