feat(transactions): prepare ERC-20 transfers before signing

[crystalt]

Adds the first client-side ERC-20 transaction lifecycle in Bedrock.

prepare_transaction_transfer constructs the exact unsigned UserOperation, requests sponsorship from pm_sponsorUserOperation (V2), and returns an opaque prepared transaction. submit_prepared_transaction signs and submits that same operation through /v2/rpc/worldchain.

This intentionally covers the sponsored happy path only. WLD self-sponsorship and its cost advisory remain follow-up work.

Stacked on #435, which provides the typed pm_sponsorUserOperation response.

---

Note

Medium Risk
Changes the public ERC-20 transfer API and submission path (V2 sponsor + send); sponsorship decline is a hard error with no fallback yet.

Overview
Splits World Chain ERC-20 transfers into a prepare → submit flow so clients can review an unsigned operation (with sponsorship applied) before signing.

prepare_transaction_transfer builds the preflight UserOperation, calls pm_sponsorUserOperation with protocol sponsorship context, merges approved paymaster/gas fields via new UserOperation::with_pm_sponsorship_approval, and returns a PreparedTransaction (UniFFI object). Declined sponsorship currently errors out (self-sponsor left as TODO).

submit_prepared_transaction checks the prepared sender matches the account, signs the operation, and submits through send_user_operation_v2. Structured logging was added for both stages.

Supporting changes: test HTTP mock handles pm_sponsorUserOperation; PmSponsorshipDeclineReason gets Display + EnumIter and a serde round-trip test; the Anvil integration test now asserts balances are unchanged until submit.

^{Reviewed by Cursor Bugbot for commit b4e1d30. Bugbot is set up for automated code reviews on this repo. Configure here.}

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-10T19:28:27.106311Z	b4e1d30	New commits

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