fix(cdk): make ecs-strategy top-of-file import hermetic vs ambient env

The inline-fallback / no-op tests in the top-of-file describe blocks assume the
OPTIONAL vars ECS_PAYLOAD_BUCKET and ECS_PLANNING_TASK_DEFINITION_ARN are ABSENT
when the module is first imported (it reads them as module-level constants). A
dev shell has neither set, so the suite passed locally — but the REAL ECS agent
container HAS ECS_PAYLOAD_BUCKET set (#502 payload bucket), so on ECS the const
was truthy and the 'no bucket → inline fallback' + 'no-op' assertions failed:

  FAIL test/handlers/shared/strategies/ecs-strategy.test.ts
    ● startSession › sends RunTaskCommand with correct params ...
    ● deleteEcsPayload without ECS_PAYLOAD_BUCKET › no-ops ...

This was the residual fork baseline-build exit-1: 2 failed / 3093 passed, only
ever reproducible inside the ECS microVM. Surfaced by the live-streaming build
log (the buffered summary had hidden it). Fix: delete both optional vars before
the top-of-file import so it's hermetic regardless of the runner's environment;
the #502/#299 describe blocks already set them via jest.isolateModules.

(cherry picked from commit 7a2bde9)
(cherry picked from commit 8267c29)

Author: isadeks

cdk/test/handlers/shared/strategies/ecs-strategy.test.ts

@@ -27,6 +27,14 @@ process.env.ECS_TASK_DEFINITION_ARN = TASK_DEF_ARN;
 process.env.ECS_SUBNETS = 'subnet-aaa,subnet-bbb';
 process.env.ECS_SECURITY_GROUP = 'sg-12345';
 process.env.ECS_CONTAINER_NAME = 'AgentContainer';
+// The top-of-file import's inline-fallback / no-op tests assume these OPTIONAL
+// vars are ABSENT at load time. They are unset in a dev shell but the real ECS
+// agent container HAS ECS_PAYLOAD_BUCKET set (#502) — so leaving this to ambient
+// env made the build pass locally yet FAIL on ECS ("works local, dies on ECS").
+// The #502 / #299 describe blocks below set these via isolateModules; delete them
+// here so the top-of-file import is hermetic regardless of the runner's env.
+delete process.env.ECS_PAYLOAD_BUCKET;
+delete process.env.ECS_PLANNING_TASK_DEFINITION_ARN;
 
 const mockSend = jest.fn();
 jest.mock('@aws-sdk/client-ecs', () => ({