perf(test): skip redundant type-check in jest via isolatedModules (#357) (#359)

The CDK Jest suite (//cdk:test) is ~91% of the CI build step (~649s of
~710s on the 4-core runner). ts-jest type-checks every file during the
test transform, duplicating the authoritative type-check already done by
//cdk:compile (tsc --build) in the same build DAG.

Switch both cdk/ and cli/ Jest transforms to a dedicated tsconfig.jest.json
that sets isolatedModules:true (transpile-only, no type-check). cdk's jest
tsconfig also pins module:CommonJS so dynamic import() downlevels to require()
(NodeNext would leave native import(), breaking jest's vm without
--experimental-vm-modules). Type-safety is unchanged: //cdk:compile and
//cli:compile remain the type-check gate.

Local timings (8-core):
  //cdk:test  314s -> 155s  (~2x)
  //cli:test  104s ->   4s  (type-check was nearly the entire CLI cost)
All 2041 CDK + 355 CLI tests pass; coverage thresholds unchanged and met.

This is an alternative to the @swc/jest approach explored on #357: same
speedup, zero test-file changes (no ES import-hoisting fallout).

Co-authored-by: bgagent <345885+scottschreckengaust@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: scottschreckengaust

cdk/package.json

@@ -122,7 +122,7 @@
       "^.+\\.[t]sx?$": [
         "ts-jest",
         {
-          "tsconfig": "tsconfig.dev.json"
+          "tsconfig": "tsconfig.jest.json"
         }
       ]
     }

cdk/tsconfig.jest.json

@@ -0,0 +1,8 @@
+{
+  "extends": "./tsconfig.dev.json",
+  "compilerOptions": {
+    "isolatedModules": true,
+    "module": "CommonJS",
+    "moduleResolution": "Node"
+  }
+}

cli/package.json

@@ -0,0 +1,6 @@
+{
+  "extends": "./tsconfig.dev.json",
+  "compilerOptions": {
+    "isolatedModules": true
+  }
+}