fix: use a valid 20-byte signer in the gasless module-sender test

[0xNilesh]

What's broken

The Unit tests job has been red on audit-fixes since 2026-08-24, failing on a single test:

--- FAIL: TestGaslessExecutePayloadWithModuleSender
    Error: Received unexpected error:
      failed to parse signer address: invalid address length for
      "cosmos1xpurwdecvsenyvpkxvmnge3cv93nyd34xuersef38pjnxen9xfsk2dnz8yek2drrv56qmn2ak9":
      got 42 bytes, want 20

Eleven commits have landed on top of it, so every audit-fix PR merged since then went in with tests red.

Why

A merge-order collision, 32 minutes wide. Neither production fix is wrong.

time	commit	what
02:04	6a1403ef	F-2026-18200 merges — adds the 20-byte signer guard in utils.GetAddressPair. That guard is the fix (module impersonation via variable-length signers).
02:36	8e28eed6	F-2026-18197 merges (#317) — brings a new test whose hardcoded signer decodes to 42 bytes.

#317's branch was cut before 18200 landed, where a 42-byte signer was still accepted. It was never re-run against the updated base, so the test went red the moment it merged.

msgServer.ExecutePayload calls utils.GetAddressPair(msg.Signer) as its first statement, so the test died there — before reaching anything it was written to exercise.

The fix

Use testSigner, the valid 20-byte account already defined in the same package (execute_payload_test.go:22), instead of the ad-hoc literal.

Worth knowing

TestGaslessExecutePayloadWithModuleSender exists to assert that gasless module-sender MsgExecutePayload still executes end to end after the x/vm VerifySender hardening. Because it was failing on its first statement, it had never once exercised that invariant. This change doesn't just turn CI green — it makes the guard real for the first time.

Verification

go test -tags="ledger test_ledger_mock test" ./test/integration/uexecutor/... -run TestGaslessExecutePayloadWithModuleSender -v
=== RUN   TestGaslessExecutePayloadWithModuleSender
--- PASS: TestGaslessExecutePayloadWithModuleSender (0.05s)

go test -tags="ledger test_ledger_mock test" ./test/integration/uexecutor/...
ok  github.com/pushchain/push-chain-node/test/integration/uexecutor  10.039s

Full ./... run with the CI invocation was in progress at time of opening; will confirm on the PR.

Follow-up for maintainers

The tests job is not blocking merges on audit-fixes — that's how eleven commits landed on a red job. While the branch is red, any new PR inherits a red tests job, so genuinely new breakage looks identical to this and has to be caught by manually diffing against clean base. Worth deciding whether the job should be required.