fix(gateway): preserve explicit delivery target ids

[alexzhu0]

Summary

• preserve explicit delivery target chat IDs verbatim instead of lowercasing the whole target string
• avoid splitting Matrix room IDs into chat_id + thread_id just because the room ID contains :
• add regression coverage for Slack case-sensitive IDs and Matrix room IDs

Verification

• source venv/bin/activate && pytest -q tests/gateway/test_delivery.py

Closes #11768

[alexzhu0]

Heads-up: #11893 (by @nightq) targets the same issue (#11768) with a different approach (case-sensitive chat-id preservation in DeliveryTarget.parse). The two diffs are not equivalent — ours focuses on explicit-id round-tripping, theirs on case handling. Fine to close this in favour of #11893 if you pick that direction; otherwise the fixes may be complementary.

[alexzhu0]

Closing as part of post-mortem cleanup of an early-batch sweep that did not get review traction. Patch still applies cleanly to current main, and issue #11768 remains open (competitor #11893 was closed without merge) — feel free to repurpose if useful. My contribution methodology has moved to alexzhu0/echo-agent.