fix(gateway): use user role for cron mirror messages to maintain message alternation

[ygd58]

Fixes #2221

Root Cause

gateway/mirror.py was appending cron delivery messages with role: "assistant". Since the previous message is also role: "assistant" (the agent's response), this created two consecutive assistant messages — violating the expected user/assistant alternation.

Fix

Changed mirror message role from "assistant" to "user" and added a [Cron delivery from {source_label}] prefix so the model understands this is an injected cron result, not a real user message.

Impact

• Fixes two consecutive assistant messages in gateway sessions
• Prevents API rejections from strict providers that require strict alternation
• Consistent with how the model should process injected cron outputs

[teknium1]

Closing — this has been superseded by recent gateway changes to cron delivery and session handling. Thanks for the contribution!

[SteelDynamite]

The issue this PR was trying to solve still exists.

See:
#8793
#21441
#10650
#21750
#7400

This is a significant issue in the UX.