fix(server): hoist system messages to front for templates requiring system-first

[rafaeldrincon]

Problem

Some chat templates (e.g. Qwen3.6) validate message order and raise:

System message must be at the beginning.

when the system message is not at index 0. This causes a 400 Bad Request for callers that send system messages in non-first positions (e.g. OpenAI-compatible API gateways that reorder messages, or multi-turn conversations where a system message appears after a user message).

Fix

render_messages() in generation.py now hoists system messages to the front of the list before returning. This is a no-op when:

• System message is already at index 0
• No system message is present

Verification

Tested with Qwen3.6-35B-A3B (NVFP4):

• Before: POST /v1/chat/completions with [user, system, user] → 400 "System message must be at the beginning"
• After: same request → 200 OK, correct response
• Also verified with tools present (system + tools array) → 200 OK

Tests

Three unit tests added to tests/server/test_mm_input.py:

• test_render_messages_hoists_system_to_front — system not first → hoisted
• test_render_messages_preserves_system_first — system already first → no-op
• test_render_messages_no_system_unchanged — no system → order preserved

Impact

• Minimal: 6 lines of logic, no new dependencies, no API change
• Backward compatible: only reorders when system messages exist and aren't first
• Multiple system messages: all grouped at front (preserves relative order among system msgs)