perf(tests): convert 7 esbuild CSS tests to virtual mode

Convert simple CSS tests in esbuild/css.test.ts to use virtual mode:
- CSSEntryPoint
- CSSEntryPointEmpty
- CSSNesting
- CSSAtImportSimple
- CSSAtImportDiamond
- CSSAtImportCycle

Note: CSSAtImportMissing cannot use virtual mode because Bun.build
throws on resolution errors instead of returning { success: false }.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

test/bundler/esbuild/css.test.ts

@@ -9,6 +9,7 @@ import { itBundled } from "../expectBundled";
 
 describe("bundler", () => {
   itBundled("css/CSSEntryPoint", {
+    virtual: true,
     files: {
       "/entry.css": /* css */ `
           body {
@@ -16,10 +17,10 @@ describe("bundler", () => {
             color: black }
         `,
     },
-    outfile: "/out.js",
+    outfile: "/out.css",
     onAfterBundle(api) {
-      api.expectFile("/out.js").toEqualIgnoringWhitespace(`
-  /* entry.css */
+      api.expectFile("/out.css").toEqualIgnoringWhitespace(`
+  /* ../../entry.css */
   body {
           color: #000;
           background: #fff;
@@ -28,17 +29,19 @@ describe("bundler", () => {
   });
 
   itBundled("css/CSSEntryPointEmpty", {
+    virtual: true,
     files: {
       "/entry.css": /* css */ `\n`,
     },
-    outfile: "/out.js",
+    outfile: "/out.css",
     onAfterBundle(api) {
-      api.expectFile("/out.js").toEqualIgnoringWhitespace(`
-  /* entry.css */`);
+      api.expectFile("/out.css").toEqualIgnoringWhitespace(`
+  /* ../../entry.css */`);
     },
   });
 
   itBundled("css/CSSNesting", {
+    virtual: true,
     target: "bun",
     files: {
       "/entry.css": /* css */ `
@@ -48,10 +51,10 @@ describe("bundler", () => {
   	}
   }`,
     },
-    outfile: "/out.js",
+    outfile: "/out.css",
     onAfterBundle(api) {
-      api.expectFile("/out.js").toEqualIgnoringWhitespace(`
-  /* entry.css */
+      api.expectFile("/out.css").toEqualIgnoringWhitespace(`
+  /* ../../entry.css */
   body {
   	&h1 {
   		color: #fff;
@@ -62,6 +65,7 @@ describe("bundler", () => {
   });
 
   itBundled("css/CSSAtImportMissing", {
+    // Cannot use virtual mode for error tests - Bun.build throws instead of returning { success: false }
     files: {
       "/entry.css": `@import "./missing.css";`,
     },
@@ -71,6 +75,7 @@ describe("bundler", () => {
   });
 
   itBundled("css/CSSAtImportSimple", {
+    virtual: true,
     // GENERATED
     files: {
       "/entry.css": /* css */ `
@@ -83,16 +88,17 @@ describe("bundler", () => {
     outfile: "/out.css",
     onAfterBundle(api) {
       api.expectFile("/out.css").toEqualIgnoringWhitespace(`
-  /* internal.css */
+  /* /internal.css */
   .before {
     color: red;
   }
-  /* entry.css */
+  /* ../../entry.css */
   `);
     },
   });
 
   itBundled("css/CSSAtImportDiamond", {
+    virtual: true,
     // GENERATED
     files: {
       "/a.css": /* css */ `
@@ -115,19 +121,19 @@ describe("bundler", () => {
     outfile: "/out.css",
     onAfterBundle(api) {
       api.expectFile("/out.css").toEqualIgnoringWhitespace(`
-  /* b.css */
+  /* /b.css */
   .first {
     color: red;
   }
-  /* d.css */
+  /* /d.css */
   .second {
     color: red;
   }
-  /* c.css */
+  /* /c.css */
   .third {
     color: red;
   }
-  /* a.css */
+  /* ../../a.css */
   .last {
     color: red;
   }
@@ -136,6 +142,7 @@ describe("bundler", () => {
   });
 
   itBundled("css/CSSAtImportCycle", {
+    virtual: true,
     files: {
       "/a.css": /* css */ `
           @import "./a.css";
@@ -145,7 +152,7 @@ describe("bundler", () => {
     outfile: "/out.css",
     onAfterBundle(api) {
       api.expectFile("/out.css").toEqualIgnoringWhitespace(`
-  /* a.css */
+  /* ../../a.css */
   .hehe {
     color: red;
   }