fix: normalize messages before chat template application

[Thump604]

Summary

• Add _normalize_messages() in server.py: maps non-standard roles (developer -> system, per OpenAI Responses API) and merges consecutive same-role messages
• Apply it in all four request paths before apply_chat_template: create_chat_completion MLLM path, create_chat_completion LLM path, create_anthropic_message, _stream_anthropic_messages
• Fixes crashes from developer role (Qwen3.5 template rejects unknown roles) and consecutive same-role messages (e.g. OpenCode sends [system, system, user, user])

Split from #224 for easier review. The other parts of #224 are in #NEW-hybrid-batching and #NEW-scheduler.

Behavior

Only merges when both adjacent messages have string content. Messages with list content (multimodal image/video payloads) are left as-is to preserve attachments.

Test plan

• Multi-turn conversation with role: "developer" messages does not crash
• OpenCode [system, system, user, user] format normalizes to [system, user]
• Well-formed alternating messages pass through unchanged
• Multimodal messages with list content are not mangled

[janhilgard]

Reviewed the diff. Clean implementation — _normalize_messages() correctly maps developer -> system and merges consecutive same-role messages with \n\n separator. Good guard on list content (multimodal payloads preserved). All 4 request paths covered (MLLM, LLM, Anthropic, Anthropic streaming). Tests are thorough — edge cases for None content, multimodal, and 3+ consecutive messages.

This fixes real crashes we see with OpenCode and agent frameworks that send [system, system, user, user] or developer role messages.

[Thump604]

Status ping — this PR has been open 7 days with no review activity. I kept the scope intentionally narrow: normalize messages before apply_chat_template so out-of-order or consecutive same-role messages do not break Qwen 3.5's chat template. This is the same message-normalization issue I hit in production earlier. The branch is mergeable on current main (d19a8d3d) and CI is green. Flagging for visibility.

[janhilgard]

Already approved on my side. CI green, branch mergeable, scope is minimal and well-contained. @waybarrios — this one is ready to go whenever you get a chance.