test(dto): pdf + support + bank account + buy payment info (+21 tests)

[TaprootFreak]

Summary

Stage 38 of the coverage push. Pure value-object + DTO file covering the PDF wire surface, the support issue ladder, BankAccount equatable contract and BuyPaymentInfo equality.

Cases

Target	Cases
`MultiReceiptDto.toJson`	2 — default CHF; renames `txIds` → `txHashes`
`SingleReceiptDto.toJson`	2 — default CHF; renames `txId` → `txHash`
`PdfDto.fromJson`	1 — reads `pdfData`
`BalancePdfDto.toJson`	2 — default EN (uppercased on wire); override + uppercased language code
`SupportIssueType`	3 — round-trip; falls back to `genericIssue`; `toJson` returns the wire value
`SupportIssueReason`	2 — round-trip; falls back to `other`
`SupportIssueState`	2 — round-trip; falls back to `created`
`SupportIssueDto.fromJson`	2 — full happy path; absent `messages` → `[]`
`SupportIssue`	2 — `isOpen` for `created` / `pending` only; `fromDto` maps fields
`BankAccount`	2 — equality by `id` only; `isActive` defaults to false
`BuyPaymentInfo`	1 — equatable props pin every field (currency-mismatch breaks equality)

What's pinned

• The two-step `txId` rename (`txId` → `txHash`, `txIds` → `txHashes`) is locked on both `SingleReceiptDto` and `MultiReceiptDto`.
• `BalancePdfDto` uppercases `Language.code` (`en` → `EN`) on the wire — a silent lowercase would break the contract.
• All three support enums use `firstWhere(... orElse: ...)` to fall back to a sentinel on unknown wire values; each fallback is pinned by name so refactors can't silently change which sentinel returns.
• `SupportIssueDto.messages` is optional on the wire and resolves to an empty list, not null.
• `BankAccount` Equatable `props` is `[id]` only — two accounts with the same id compare equal even if name / iban / isActive differ.
• `BuyPaymentInfo` props enumerate every field — the negative case (currency difference) is the canary.

Test plan

• `flutter test test/packages/service/dfx/models/pdf_support_bank_test.dart` — 21 pass
• `flutter analyze` clean on the new file
• CI green