Improve test suite to avoid flaky container event order

[clue]

This changeset improves the test suite to avoid a flaky container event order. The test container could still be running when the test attached to it, so the die and attach events arrived in either order and failed 5 of the last 17 CI runs (counting reruns). The test now waits for the container to exit first, which also means it is never killed on removal, so the separate expectation for Docker before 20.10 (2020) is no longer needed. The race dates back to #61, the library itself is unchanged.

Builds on top of #98, #80 and #61

[Copilot]

🟢 Approval recommended

The focused test changes address the event-order race with no blocking issues identified.

No open issues 🎉

What changed in this PR

Stabilizes the container lifecycle test by waiting for exit before attaching, removing the event-order race without changing the library.

Changes:

• Waits for container exit and verifies a zero exit status.
• Replaces version-dependent event expectations with one ordered sequence.

File	Description
tests/​FunctionalClientTest.php	Waits for exit before attaching and simplifies event assertions.

_{Review effort: Balanced}