fix: update test-runner-mocha rollup config for rewriteRelativeImportExtensions

TypeScript's rewriteRelativeImportExtensions injects a
__rewriteRelativeImportExtension helper into the output. This helper
is not needed in browser bundles and breaks test execution.

Add stripRewriteImportExtensionPlugin to remove the helper from the
rollup output. Also add resolveMochaPlugin for robust monorepo
module resolution.

Assisted-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: bennypowers

packages/test-runner-mocha/rollup.config.mjs

@@ -1,6 +1,9 @@
+import { createRequire } from 'module';
 import deepmerge from 'deepmerge';
 import createConfig from '../../rollup.browser.config.mjs';
 
+const require = createRequire(import.meta.url);
+
 const REGEXP_DTS_MOCHA = /'..\/..\/..\/node_modules\/mocha\/mocha.js'/g;
 const REGEXP_DTS_CORE = /'..\/..\/test-runner-core\/browser\/session.js'/g;
 
@@ -16,6 +19,51 @@ const rewriteDtsPlugin = {
   },
 };
 
+const stripRewriteImportExtensionPlugin = {
+  name: 'strip-rewrite-import-extension',
+  renderChunk(code) {
+    // Remove the __rewriteRelativeImportExtension helper and its calls
+    // that TypeScript injects when rewriteRelativeImportExtensions is enabled
+    code = code.replace(/var __rewriteRelativeImportExtension[^;]*;/g, '');
+    // Replace calls with their argument, handling nested parentheses
+    const fnName = '__rewriteRelativeImportExtension(';
+    let result = '';
+    let i = 0;
+    while (i < code.length) {
+      const idx = code.indexOf(fnName, i);
+      if (idx === -1) {
+        result += code.slice(i);
+        break;
+      }
+      result += code.slice(i, idx);
+      // Find the matching closing paren
+      let depth = 1;
+      let j = idx + fnName.length;
+      while (j < code.length && depth > 0) {
+        if (code[j] === '(') depth++;
+        else if (code[j] === ')') depth--;
+        j++;
+      }
+      // Extract the argument (everything between the outer parens)
+      result += code.slice(idx + fnName.length, j - 1);
+      i = j;
+    }
+    return result;
+  },
+};
+
+const resolveMochaPlugin = {
+  name: 'resolve-mocha',
+  resolveId(id) {
+    // Resolve mocha using Node's module resolution from this package,
+    // rather than relative path traversal which may land in the wrong
+    // node_modules in a monorepo
+    if (id.endsWith('node_modules/mocha/mocha.js')) {
+      return require.resolve('mocha/mocha.js');
+    }
+  },
+};
+
 const rewriteWebSocketImportPlugin = {
   resolveId(id) {
     if (id === '/__web-dev-server__web-socket.js') {
@@ -36,7 +84,7 @@ export default [
         'wds-socket': '/__web-dev-server__web-socket.js',
       },
     },
-    plugins: [rewriteDtsPlugin, rewriteWebSocketImportPlugin],
+    plugins: [resolveMochaPlugin, rewriteDtsPlugin, rewriteWebSocketImportPlugin, stripRewriteImportExtensionPlugin],
   }),
   deepmerge(createConfig('src/standalone.ts'), {
     output: {
@@ -46,6 +94,6 @@ export default [
         'wds-socket': '/__web-dev-server__web-socket.js',
       },
     },
-    plugins: [rewriteDtsPlugin, rewriteWebSocketImportPlugin],
+    plugins: [resolveMochaPlugin, rewriteDtsPlugin, rewriteWebSocketImportPlugin, stripRewriteImportExtensionPlugin],
   }),
 ];