Write tool result for new files renders without added-line (green) highlighting, unlike Edit

Describe the bug
In the chat stream, an Edit tool result renders as a diff with green added-line highlighting. A Write tool result creating a new file renders the file contents as plain, unhighlighted lines — even though every line is an addition. Inconsistent presentation for what is semantically the same thing (added lines).

To reproduce

1. Ask Claude to create a new file (Write tool) — result renders with no green highlighting.
2. Ask Claude to modify an existing file (Edit tool) — result renders as a diff with green added lines.

Expected behavior
Write results for new files render with the same added-line (green) treatment as Edit diffs — a new file is a 100%-additions diff.

Related (distinct) issues

• #67984 — MCP-provided Edit/Write tools get no diff rendering at all (this report is about the built-in Write tool)
• #37951 — feature request to hide inline diffs (this report is about consistency when they're shown)

Custom themes can recolor diff lines (diffAdded etc.), but no theme token or settings.json knob controls whether a Write result renders as a diff — that per-tool choice is hardcoded in the client.