fix(client): support legacy wallet gasless execution

[cesarenaldi]

Summary

• Estimate Proxy wallet gas for the RelayHub signature and use the relayer-provided relay address.
• Replace mocked gasless payload assertions with live generic Proxy and Safe legacy wallet gasless execution coverage.
• Reuse the deposit wallet private key fixture when constructing the deposit wallet signer.

Verification

• pnpm exec vitest --project client-integration --run packages/client/tests/integration/gasless.test.ts
• pnpm exec vitest --project client-integration --run packages/client/tests/integration/approvals.test.ts
• pnpm exec vitest --project client --run packages/client/src/rpc.test.ts
• pnpm lint
• pnpm typecheck

---

Note

Medium Risk
Changes signed RelayHub fields and relay address for Proxy gasless txs, which can break relayer validation if estimates or params are wrong; live integration tests increase env/credential dependence but do not alter production auth paths for deposit wallets.

Overview
Fixes legacy Proxy gasless signing so RelayHub parameters match what the relayer expects.

Proxy workflow now sets gasLimit from eth_estimateGas on the encoded factory call (replacing a fixed placeholder) and uses the relayer’s executeParams.address as the relay in the signed hash instead of the zero address.

Adds JsonRpcClient.ethEstimateGas (with a unit test) to support that estimation.

Integration tests drop MSW submit-shape assertions in favor of live end-to-end gasless runs for Proxy and Safe wallets via approveErc20 and wait(), using builder API auth. The deposit wallet signer fixture is wired through the shared private-key fixture.

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