#2285 fix(serve): preserve late instruction roles across adapters

Descrição

Late system/developer messages currently become user messages on the Chat, Messages and Responses paths. Their position survives, but their instruction role does not.

Preserve the role through adapters and render late system/developer content as system messages in the bundled template. Keep leading instruction merging and the existing rejection of images in system content. Remove the conversion helper and its imports.

Based on main `a0116b2782a5b82622358c04a06496485d12b2d6`. Related #56 fixed template refusal by changing late roles to user; this change retains the accepted messages' instruction role instead. Custom templates remain responsible for their own accepted roles.

Validation: 52 focused model-free client/Responses tests pass. The strengthened existing HTTP tests fail on stock for the missing system-role markers and pass after this change. They verify order and one occurrence of each late instruction. No new GPU workload or deployment, and no model-quality or throughput claim.

Command: `python3 -m unittest serve.test_server.ClientShapes serve.test_responses`.