fix(config): keep embedded-constructed structs constructible

[polaz]

The blanket non_exhaustive broke embedded consumers (coordinode) that build ProxyConfig + wiring structs + DescriptorSource::Embedded with runtime values. Scope non_exhaustive to the leaf (deserialized-only) config structs; keep the wiring structs constructible. Add from_yaml_str + a separate-crate integration test guarding the contract.

[coderabbitai]

🔗 Related PRs

#44 - fix(config): mark config structs non_exhaustive + center donation section [merged]

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