- What I did

I migrated TestContainerAPIPostContainerStop to integration tests.
Relates to #50159

- How I did it

• Migrated TestContainerAPIPostContainerStop to integration/container/stop_test.go from integration-cli/docker_api_containers_test.go.
• Changed from apiClient calls to direct HTTP requests because,
  • apiClient usage is already covered in other test. (e.g. TestContainerAPIStop)
  • the test targets the route handler behavior, as indicated by its name.
• Added subtests for scenarios where the container is already stopped and where the container does not exist.

- How to verify it

Verified running the integration test with following command.
TEST_FILTER='TestContainerAPIPostContainerStop' TEST_SKIP_INTEGRATION_CLI=1 make test-integration

- Human readable description for the release notes

- A picture of a cute animal (not mandatory but encouraged)