test(sell-service): confirmPayment software wallet happy path (+2 tests)

[TaprootFreak]

Summary

Stage 59 of the coverage push. Drives `confirmPayment` end-to-end with a real `EthPrivateKey` credential so the signing chain (`Eip712Signer.signDelegation` + `Eip7702Signer.signAuthorization`) is exercised, not just the validation guard.

Cases

Case	What's pinned
Software wallet happy path	PUTs to `/v1/realunit/sell//confirm`; body carries the `eip7702` envelope (not the `txHash` branch); delegation block mirrors the eip7702 data + carries a 65-byte real signature; authorization block carries the matching chain / nonce / (r, s, yParity)
4xx response	`ApiException` propagates from `_sendConfirm`

What's pinned

• The delegation `salt` is serialised as a string (`'0'`), not the raw int. Pinned via string equality so a regression to numeric serialization surfaces.
• The authorization block uses `chainId` from `domain.chainId`, `address` from `delegatorAddress` (the MetaMask Delegator contract), and `nonce` from `userNonce` — the test asserts each value explicitly.
• The signature length pin (132 chars including `0x`) catches a serialization regression that drops the trailing `v` byte.
• Deterministic test key is shared with `FakeBitboxCredentials`. Reusing it keeps test infrastructure consistent and lets future BitBox-vs-software comparison tests reuse the same address.

Test plan

• `flutter test test/packages/service/dfx/real_unit_sell_payment_info_service_confirm_test.dart` — 2 pass
• `flutter analyze` clean on the new file
• CI green