fix(coding-agent): restore diff code block highlighting

closes #5092

packages/coding-agent/test/syntax-highlight.test.ts

@@ -1,4 +1,6 @@
-import { describe, expect, it } from "vitest";
+import { resetCapabilitiesCache, setCapabilities } from "@earendil-works/pi-tui";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { highlightCode, initTheme } from "../src/modes/interactive/theme/theme.ts";
 import { highlight, renderHighlightedHtml, supportsLanguage } from "../src/utils/syntax-highlight.ts";
 
 describe("syntax highlight renderer", () => {
@@ -46,3 +48,29 @@ describe("syntax highlight renderer", () => {
 		expect(rendered).toContain("[number:1]");
 	});
 });
+
+describe("theme syntax highlighting", () => {
+	beforeEach(() => {
+		setCapabilities({ images: null, trueColor: true, hyperlinks: false });
+		initTheme("dark");
+	});
+
+	afterEach(() => {
+		resetCapabilitiesCache();
+	});
+
+	it("colors diff additions and deletions in fenced diff blocks", () => {
+		const lines = highlightCode("-old\n+new\n", "diff");
+
+		expect(lines[0]).toBe("\x1b[38;2;204;102;102m-old\x1b[39m");
+		expect(lines[1]).toBe("\x1b[38;2;181;189;104m+new\x1b[39m");
+	});
+
+	it("keeps cli-highlight default styled scopes mapped to theme styles", () => {
+		expect(highlightCode("const re = /foo+/gi;", "javascript")[0]).toContain(
+			"\x1b[38;2;206;145;120m/foo+/gi\x1b[39m",
+		);
+		expect(highlightCode("@decorator", "python")[0]).toBe("\x1b[38;2;128;128;128m@decorator\x1b[39m");
+		expect(highlightCode("<div></div>", "html")[0]).toContain("\x1b[38;2;86;156;214mdiv\x1b[39m");
+	});
+});