fix(gateway): preserve explicit delivery target IDs in delivery parsing

[Junass1]

Summary

This fixes DeliveryTarget.parse() so explicit delivery targets preserve the original ID payload instead of lowercasing and mis-splitting it.

Before this change:

• slack:C123ABC was parsed as chat_id="c123abc"
• matrix:!RoomABC:example.org was parsed as chat_id="!roomabc" and thread_id="example.org"
• matrix:@Alice:example.org was parsed the same lossy way

After this change:

• only the platform token is normalized
• explicit target payloads are preserved verbatim
• Matrix room/user IDs keep their full homeserver-qualified form
• existing unambiguous thread syntax like telegram:12345:678 still works

Why

DeliveryTarget.parse() was lowercasing the entire target string and then using split(":", 2) for every platform. That works for simple numeric targets, but it breaks:

• case-sensitive channel IDs like Slack channel IDs
• platform IDs that legitimately contain :, especially Matrix room/user IDs

This is a small gateway bugfix with direct impact on cron delivery and explicit cross-platform routing.

Changes

• updated gateway/delivery.py to normalize only the platform segment
• preserved the explicit target payload as-is
• treated Matrix !room:server and @user:server targets as full chat_id values instead of interpreting the homeserver colon as a thread separator
• added regression coverage in tests/gateway/test_delivery.py

Tests

Passed:

• python -m pytest tests/gateway/test_delivery.py -q
• python -m pytest tests/gateway/ -k delivery -q

[teknium1]

Thanks for the contribution, @Junass1!

Closing this as a duplicate of #11962 (by @alexzhu0), which targets the same fix/feature. We're consolidating on that PR for review.

If you want to help push it over the line, please jump in there — or if you think your approach is better for a specific reason that isn't covered in the other PR, let us know and we can reopen.