Allow YAML structured help text in edge app settings

[renatgalimov]

Summary

• allow edge app setting help_text to deserialize from YAML mappings and re-emit structured descriptors when saving manifests
• refresh manifest tests and utilities to cover structured help_text parsing/serialization and drop unused serde helpers
• update the Edge Apps documentation to highlight the YAML descriptor format instead of embedded JSON strings

Testing

• cargo test

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https://chatgpt.com/codex/tasks/task_e_68e6a423ee8c8330b1948b1ad3db95ce