refactor: trim restating comments and tighten test variable names

[tiana-code]

Full code-quality pass requested by the owner, covering comments, naming, and structure.

Findings

• Structure is already correct. Every service keeps clean hexagonal layers (thin controllers with no @Transactional, interface + impl services, DTOs split into request/response, enums under domain/enum, repositories in infrastructure). Nothing to change.
• Production naming is already clean. No cryptic one or two letter names in main code. The only offenders were a few single-letter locals in tests, now renamed (c/s/a to case/session/alert, and a/b to debitAccount/creditAccount).

Comment changes (balanced bar)

• Cut docstrings that restated adjacent code, the sandbox provider doc blocks first (the marker-to-result narration the when block already shows), plus a handful of restating one-liners.
• Trimmed verbose blocks to the non-obvious why.
• Kept comments carrying a real invariant: transaction-boundary rules, fail-closed and constant-time security notes, concurrency and optimistic-lock semantics, MapStruct limitations, privacy/opaque-token notes, and the public plug-in port contracts (KycProvider, SanctionsProvider, AmlCopilot, BankProvider and their types), which document the adapter API for OSS adopters and back PROVIDERS.md.

17 files, comments and test names only, no behavior change. Verified: all four services compile; compliance/ledger/decision unit tests green (466 total); spotless and detekt clean. Integration tests run on CI (need Docker).