Add wire types for runtime agent registration (#55398)

## What changed

Add `codex-runtime-agent-instance` with serializable registration requests and deserializable responses. Requests support OAuth, personal access tokens, and platform API keys, along with an installation ID, public key, harness metadata, and runtime role. Responses include runtime and host IDs plus ChatGPT or platform actor context.

Use `RedactedString` for credentials to keep secrets out of debug output.

## Testing

Add tests covering request serialization for all three credential types, debug redaction, response decoding for both actor types, and rejection of responses missing required fields.

GitOrigin-RevId: f33bae734fdd1402144dee407e5b868f5eef1d37

codex-rs/runtime-agent-instance/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+name = "codex-runtime-agent-instance"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[lib]
+doctest = false
+
+[lints]
+workspace = true
+
+[dependencies]
+codex-utils-redacted-string = { workspace = true }
+serde = { workspace = true, features = ["derive"] }
+uuid = { workspace = true, features = ["serde"] }
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }
+serde_json = { workspace = true }