exec-server: expose environment registry payloads

[viyatb-oai]

Why

Services that proxy the exec-server environment registry endpoints need to deserialize and forward the same Noise registration and harness-key validation payloads. Those wire models currently live as private, serialize-only structs in exec-server, which forces consumers to duplicate the contract.

What changed

• Add owned serde models for registration and harness-key validation requests and responses.
• Use those models in the existing exec-server registry client.
• Re-export the models from codex-exec-server and codex-core-api.
• Keep the harness authorization request free of a derived Debug implementation so it is not accidentally logged.

Testing

• Focused exec-server registration and harness-key validation tests: 2 passed.
• cargo check -p codex-core-api

The full codex-exec-server suite compiled and ran 254 tests: 222 passed, while 32 existing filesystem sandbox tests could not run under the nested macOS sandbox (sandbox_apply: Operation not permitted).