5/5: Tests and changelogs for External-IVK-only scanning

[czarcas7ic]

Summary

• Add TestState::enhance_transaction() and service_enhancement_requests() helpers that mirror the production enhancement pipeline more closely
• Store raw transaction bytes in TestState so serviceable enhancement requests are replayed from a production-like raw-transaction source instead of depending on wallet-visible raw state
• Tighten service_enhancement_requests() so it fails on missing raw bytes for serviceable enhancement work and detects stabilized serviceable queues, while still tolerating compact-only enhancement requests that the harness cannot reconstruct
• Tighten enhance_transaction() so bundle positions are only derived when the tx is actually present in the cached compact block; otherwise it degrades or fails cleanly instead of fabricating TxBundlePositions
• Rewrite change-note tests to create Internal-scope notes via real spend + enhancement instead of direct injection
• Tighten the ZIP 315 confirmation-policy assertions and the v_transactions_hides_unenhanced_txs funding-transaction check
• Add enhancement_cascade_survives_pruning, v_transactions_hides_unenhanced_txs, and pruning_wipes_late_discovered_spend_locator tests
• Update existing tests to call service_enhancement_requests() after generating blocks

Context

This is PR 5/5 — tests proving the full system works end-to-end. The changelog ownership was pushed down into the earlier branches during the fix round, so this final slice is now focused on test harness fidelity and end-to-end regressions.

Full stack:

1. adam/enriched-decrypted-output — API foundations
2. adam/sync-enhancement-pipeline — Core behavioral change
3. adam/sqlite-enhancement-support — SQLite backend implementation
4. adam/memory-backend-enhancement — Memory backend implementation
5. This PR — End-to-end tests

Review focus

• enhance_transaction() helper fidelity relative to the production store_raw_transaction path
• The raw-transaction registry used by service_enhancement_requests() and how it distinguishes serviceable enhancement requests from compact-only ones
• The tightened ZIP 315 and transaction-history assertions, especially on the SQLite backend

Test plan

• cargo check -p zcash_client_backend --all-features
• cargo test -p zcash_client_sqlite --features "sync orchard" can_spend_inputs_by_confirmations_policy
• cargo test -p zcash_client_sqlite --features "sync orchard" v_transactions_hides_unenhanced_txs
• cargo test -p zcash_client_sqlite --features "sync orchard" enhancement_cascade_survives_pruning
• cargo test -p zcash_client_memory --all-features