fix(bridge): resolve type errors in bridge-sandbox-e2e suite

[islandbitcoin]

Resolves the Check Code (tsc) failures surfaced once CI began running on the integration branch. The test/flash/bridge-sandbox-e2e/ suite was never type-checked while Actions was disabled, so type errors accumulated. Test-only changes — no production code touched. Fixes ENG-423.

Changes

• helpers.ts — mock req/res casts (TS2352): the inject helpers cast createReqRes()'s generic objects straight to the Express handler param types. Route them through as unknown as … (TS's own suggestion).
• helpers.ts — execQuery (TS2339): made generic (<T = Record<string, unknown>>, backward-compatible default) so call sites can type the GraphQL payload.
• helpers.ts — HandlerResponse.body (TS18046): typed as Record<string, unknown> instead of unknown (defaulting to {} in the inject returns), which fixes .body access in deposit-withdrawal.spec and external-account.spec without per-site casts.
• cutover-state.spec.ts: type the execQuery result and narrow the error union (if ("errors" in response) throw …) before asserting.

Validation caveat

• I could not run tsc locally (no install in my environment), so this is analysis-validated against the exact errors tsc reported. CI is the real check.
• This PR won't show checks until ci: run checks on the bridge-rebase integration branch #408 merges — the integration-branch CI trigger isn't active yet, so PRs into tmp/bridge-rebase-pr-ready don't run workflows. Either merge ci: run checks on the bridge-rebase integration branch #408 first, or validate this alongside it.
• Scoped strictly to Check Code / the e2e suite. The other red checks (Redis service, integration config, vendir, bats) are separate infra issues tracked in ENG-422.

🤖 Generated with Claude Code

[linear]

ENG-423