Fail SMS send responses that omit the +CMGS submit reference

[Justinabox]

Summary

SMSService.send() reports an SMS as sent when the modem response contains only OK and no +CMGS: <mr> submit reference. The returned SMS is saved with reference=0, which is indistinguishable from a legitimate TP-MR value of zero and breaks delivery-report correlation/debugging.

Evidence

Affected code:

• callstack/sms/service.py:155-161 treats send_data(..., expect=["+CMGS:", "OK"]) as successful when the final OK is seen.
• callstack/sms/service.py:163-169 initializes reference = 0 and leaves it at zero if no +CMGS: line is present.
• callstack/sms/service.py:171-180 then saves/emits a sent SMS with reference=0.
• callstack/protocol/executor.py:302-312 uses exact success matching, so +CMGS: 42 is not actually the success terminator; the later OK is what makes the send successful.

No-hardware reproduction from main (6926b369cdba):

PYTHONPATH=. uv run --no-project --with pyserial-asyncio python - <<'PY'
import asyncio
from callstack.events.bus import EventBus
from callstack.protocol.executor import ATCommandExecutor
from callstack.protocol.urc import URCDispatcher
from callstack.sms.service import SMSService
from callstack.transport.mock import MockTransport

async def main():
    bus = EventBus()
    transport = MockTransport()
    service = SMSService(ATCommandExecutor(transport, URCDispatcher(bus)), bus)
    transport.feed('> ')
    transport.feed('OK')
    sms = await service.send('5551234', 'Hello')
    print('sms_reference:', sms.reference)
    print('sms_status:', sms.status)
    print('written_commands:', [w.strip() for w in transport.all_written])

asyncio.run(main())
PY

Actual output:

sms_reference: 0
sms_status: sent
written_commands: ['AT+CMGS="5551234"', 'Hello\x1a']

Expected behavior

For text-mode AT+CMGS, Callstack should require an explicit parsed +CMGS: <mr> line before reporting a successful sent SMS. Absence of the submit reference should be treated as a send/protocol error, not silently normalized to 0.

Important nuance: +CMGS: 0 can be a legitimate modem reference, so the fix should use a sentinel such as reference: int | None while parsing rather than using 0 as both "missing" and "real zero".

Suggested fix direction

• Parse +CMGS: into reference: int | None = None.
• After a successful final OK, raise SMSSendError if reference is None.
• Preserve valid +CMGS: 0 as a real successful reference.
• Add focused tests for both OK without +CMGS and explicit +CMGS: 0.

Acceptance criteria

• send() raises SMSSendError when the submit response lacks +CMGS: <mr> even if it ends in OK.
• send() still accepts and returns reference=0 when the modem explicitly returns +CMGS: 0.
• Sent-message persistence and SMSSentEvent emission happen only after an explicit reference is parsed.
• Existing SMS prompt/body timeout and GSM 03.38 encoding tests remain green.

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_sms_service.py -q
PYTHONPATH=. uv run --no-project --with pytest --with pytest-asyncio --with pytest-aiohttp --with pyserial-asyncio --with aiosqlite pytest tests/ -q

Duplicate check

Searched existing issues/PRs for CMGS missing reference, SMS send reference 0, and OK no +CMGS. Existing issue #13 covered exact final-result matching for SMS body truncation, not submit-reference absence. I did not find an existing open issue or PR for this behavior.