Make activeNodeIDs order predictable and fix broken test

[mkysel]

Sort active node IDs in pkg/payerreport/generator.go::extractActiveNodeIDs to make ordering predictable and return the last submission error from pkg/payerreport/workers/submitter.go::SubmitterWorker.SubmitReports to fix the broken test

This change introduces deterministic ordering for active node IDs and updates report submission error handling to return the last encountered error after processing all reports.

• Update extractActiveNodeIDs to sort collected node IDs in ascending order after population in generator.go
• Modify SubmitterWorker.SubmitReports to track and return the most recent submission error instead of always returning nil in submitter.go

📍Where to Start

Start with extractActiveNodeIDs in generator.go, then review SubmitterWorker.SubmitReports in submitter.go.

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Macroscope summarized 703b30f.

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[mkysel]

Before sorting:

2025-10-08T16:41:20.903-0400    DEBUG   api     api/server.go:225       closed
--- FAIL: TestCanGenerateAndAttestReport (4.60s)
    store.go:50: creating database test_setupmultinodetest_fqqnzsrdbjnx ...
    store.go:50: creating database test_setupmultinodetest_kxviqfmewiqj ...
    integration_test.go:508: 
                Error Trace:    /Users/martinkysel/work/xmtpd/pkg/payerreport/workers/integration_test.go:508
                Error:          Received unexpected error:
                                InsufficientSignatures(uint8 validSignatureCount, uint8 requiredSignatureCount)
                Test:           TestCanGenerateAndAttestReport
FAIL
FAIL    github.com/xmtp/xmtpd/pkg/payerreport/workers   5.403s

After sorting:

go_test_clean ./pkg/payerreport/workers/... -run TestCanGenerateAndAttestReport -failfast -count 1000 -cpu 1
<no failure>