fix(manager): allow container API helpers in strict mode

[samzong]

Summary

Fix manager/scripts/lib/container-api.sh so _api and _api_code can be called without a request body when sourced from scripts running with set -u.

Previously, calls like _api GET /version or _api_code POST /containers/<name>/start could fail with an unbound variable error because $3 was accessed directly.

Fixes #188

Changes

• default the optional data argument in _api and _api_code with ${3-}
• add a regression test covering strict-mode usage without a request body
• record the manager-side change in changelog/current.md

Why

Several container API helpers intentionally omit the request body argument. That works in normal shell mode, but breaks in bash strict mode (set -u), which some manager scripts use.

Testing

• bash manager/tests/test-container-api-strict-mode.sh
• bash -n manager/scripts/lib/container-api.sh
• bash -n manager/tests/test-container-api-strict-mode.sh
• bash -n manager/agent/skills/worker-management/scripts/lifecycle-worker.sh

[CLAassistant]

All committers have signed the CLA.

[samzong]

@johnlanni it's seem not PR reason.