fix(envoy-client): release global handle after shutdown

[DevChiniwala]

• Release the cached global Envoy handle after shutdown cleanup.
• Add regression coverage that verifies the callback context is released.

Fixes #5766

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[the-company-company]

✅ No issues found

_{Reviewed commit 49f2fb2.}

[DevChiniwala]

The Co review passed with no findings. GitHub has not run the latest-head Rust CI because it requires maintainer approval: Rust CI. Could a maintainer approve that run so the Envoy lifecycle regression can be checked in CI? The cached handle is cleared only if it still belongs to the stopping instance, before the stopped notification is published.