Implement request classification, tool result merging, compact detection,
deterministic request IDs, and warmup downgrade for Copilot proxy.
The root cause was x-initiator being hardcoded to "user", making Copilot
count every API request (including tool callbacks and agent continuations)
as a separate premium interaction. The optimizer dynamically classifies
requests as "user" or "agent" based on message content analysis.
Closes#1813
Jason