fix: reduce proxy-induced continuation bias in translated turns

[edxeth]

Summary

• Remove translator behaviors that bias prompts toward continuation-style outputs by trimming assistant-prefill history and tightening conversation anchoring.
• Add request-shape validation for both /v1/messages and /v1/chat/completions to reject assistant-prefill final turns and empty user-context payloads early.
• Keep tool-result continuation explicit but neutral (Tool results:), and add regression tests for synthetic-anchor ID behavior and validator coverage.

Verification

• go test ./...
• go build ./...