Avoid incorrect SMS delivery-status updates when TP-MR repeats

[Justinabox]

Problem

SMSStore can attach a delivery report to the wrong outbound SMS when a modem reuses a TP-MR / +CMGS reference for the same recipient.

A delivery report currently contains only reference, recipient, and normalized status when it reaches SMSStore. SMSStore._latest_matching_message() then scans in reverse and selects the newest stored SMS whose (reference, recipient) matches. Since modem message references are finite and can repeat, a delayed receipt for an older outbound message can mark a newer same-recipient message as delivered or failed.

This creates incorrect delivery history for unattended notification/MFA integrations. It is preferable to leave a report uncorrelated than to mutate the status of a different logical SMS.

Reproduction (current main 6dd4211173857e6da55e6e2d0984c06fc56f9a7c)

PYTHONPATH=. uv run --no-project --with pytest --with pytest-asyncio --with pytest-aiohttp --with pyserial-asyncio --with aiosqlite python - <<'PY'
import asyncio
from callstack.sms.store import SMSStore
from callstack.sms.types import DeliveryReport, SMS

async def main():
    store = SMSStore()
    older = await store.save(SMS(
        recipient="synthetic-recipient", body="older", status="sent", reference=7
    ))
    newer = await store.save(SMS(
        recipient="synthetic-recipient", body="newer", status="sent", reference=7
    ))
    report = await store.save_delivery_report(DeliveryReport(
        reference=7, recipient="synthetic-recipient", status="delivered"
    ))
    print(report.message_id, [(m.id, m.status) for m in await store.list()])

asyncio.run(main())
PY

Observed output:

2 [(1, 'sent'), (2, 'delivered')]

The report is unconditionally attached to the newer message. This behavior is also codified in tests/test_sms_store.py::test_save_delivery_report_correlates_latest_matching_outbound_sms.

Root cause

• callstack/sms/service.py::_parse_cmgr_status_report() discards the SCTS/discharge-time fields from the text-mode +CMGR delivery report.
• callstack/sms/store.py::SMSStore._latest_matching_message() treats (reference, recipient) as unique and returns the last matching record.
• callstack/sms/store.py::_save_delivery_report_locked() assigns that result to report.message_id and updates the chosen message's status.

Suggested fix direction

Make status correlation fail closed when the (reference, recipient) key is ambiguous. A small safe first slice could:

1. Preserve parsed report timestamps where available and retain outbound submission timestamps.
2. Correlate only a unique, temporally plausible candidate; otherwise persist the delivery report with message_id=None and do not update an outbound message status.
3. Keep the existing unique-match path and the public delivery-report event behavior intact.

Do not use insertion order as a proxy for modem/carrier correlation.

Acceptance criteria

• A unique matching outbound SMS is still linked and receives the delivery status.
• Two unresolved outbound SMS records with the same recipient and reference do not cause a delayed receipt to update the newer record solely because it was inserted last.
• Ambiguous reports remain durably listed for operator inspection, with message_id=None (or an explicit documented ambiguous state), and no false outbound status mutation.
• If timestamp-based disambiguation is implemented, boundary tests cover late receipts and reject implausible matches rather than guessing.
• In-memory and SQLite-backed stores preserve the chosen behavior across reopen.
• Tests use synthetic values only and error/log paths remain PII-safe.

Affected files

• callstack/sms/service.py
• callstack/sms/store.py
• callstack/sms/types.py (only if correlation metadata needs extension)
• tests/test_delivery_reports.py
• tests/test_sms_store.py

Verification gates

git diff --check
PYTHONPATH=. uv run --no-project --with pytest --with pytest-asyncio --with pytest-aiohttp --with pyserial-asyncio --with aiosqlite pytest tests/test_delivery_reports.py tests/test_sms_store.py -q
PYTHONPATH=. uv run --no-project --with pytest --with pytest-asyncio --with pytest-aiohttp --with pyserial-asyncio --with aiosqlite pytest tests/ -q

Non-goals

• Changing modem message-reference allocation.
• Aggregating multipart segment receipts.
• Retrying delivery reports or changing SIM-slot cleanup semantics.