attempt fix of strictest tsconfig

eng/tools/specs-model/tsconfig.json

@@ -2,7 +2,7 @@
   "extends": [
     "../tsconfig.json",
     // [strictest]
-    "@tsconfig/strictest/tsconfig.json"
+    "../tsconfig_strictest.json"
   ],
   "compilerOptions": {
     "outDir": "./dist",

eng/tools/tsconfig_strictest.json

@@ -0,0 +1,28 @@
+// Copy-pasted from https://www.npmjs.com/package/@tsconfig/strictest
+// Because including with "extends" didn't work in CI. Got following error:
+//   Error: tsconfig.json(5,5): error TS6053: File '@tsconfig/strictest/tsconfig.json' not found.
+// See https://github.com/Azure/azure-rest-api-specs/actions/runs/9934716074/job/27439571615
+{
+  "compilerOptions": {
+    "strict": true,
+    "allowUnusedLabels": false,
+    "allowUnreachableCode": false,
+    "exactOptionalPropertyTypes": true,
+    "noFallthroughCasesInSwitch": true,
+    "noImplicitOverride": true,
+    "noImplicitReturns": true,
+    "noPropertyAccessFromIndexSignature": true,
+    "noUncheckedIndexedAccess": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+
+    "isolatedModules": true,
+
+    "checkJs": true,
+
+    "esModuleInterop": true,
+    "skipLibCheck": true
+  },
+  "$schema": "https://json.schemastore.org/tsconfig",
+  "_version": "2.0.0"
+}