test(functions): resolve flaky test failures from Docker exec cleanup race condition

[mandarini]

Description

Fixes flaky test failures in functions-js caused by a race condition during Docker container cleanup in CI.

Problem

Tests were intermittently failing in CI with "no such exec instance" errors during teardown. The issue occurred when testcontainers tried to clean up exec processes after the container had already stopped, causing the test suite to fail even though all actual tests passed.

Solution

• Added a safe stop() method to the Relay class that catches and ignores "no such exec" errors during cleanup
• Updated all test files to use the new relay.stop() method instead of direct container.stop()
• Changed runRelay to return a proper Relay instance instead of a plain object