test(ops): cover the submission and polling guards in extendViaFootprint

[0dillon]

Summary

extendViaFootprint has four guards that only fire against a live network: the empty-key-list
refusal, the isSimulationRestore refusal, the status !== "PENDING" submission rejection, and the
status !== "SUCCESS" post-poll failure. Only the first is reachable from a unit test today.

test/futility.test.ts establishes the pattern this needs — a structural object cast to
rpc.Server that answers the calls under test and fails loudly on the ones that should not happen.
The guards it does not cover are precisely the ones that decide whether an operator learns why a
transaction failed.

The isSimulationRestore guard matters most. It exists because ExtendFootprintTTLOp cannot
restore an archived entry — unlike InvokeHostFunctionOp, which restores automatically since
Protocol 23 — so Path B must refuse rather than submit something that cannot work. Nothing currently
proves it refuses.

Acceptance Criteria

• A fake server drives extendViaFootprint through each guard.
• The isSimulationRestore path asserts that no transaction was submitted, not just that it
  threw.
• The TRY_AGAIN_LATER and DUPLICATE submission statuses are covered, and the test asserts
  polling is never reached — the point of that guard is not burning the poll budget on a hash
  the network does not have.
• The FAILED poll outcome asserts the result code reaches the error message.
• Each test is verified by mutation: break the guard, confirm the test fails, restore it. Say in
  the pull request which mutations you ran.

Tech Stack

TypeScript 5.7 on Node 22+ (CI runs 22 and 24), ESM with moduleResolution: "NodeNext" — relative
imports need the .js extension. @stellar/stellar-sdk is pinned to an exact version; verify RPC
API shape against the installed copy, not against documentation. Tests are Vitest under test/ and
run without a network. npm run format:check, npm run lint, npm run build and npm test must
all be clean.