feat(phase-5): WS1 — Rust SDK

Implements the sentryagent-idp Rust SDK crate (sdk-rust/) with:
- TokenManager with Arc<Mutex<TokenCache>> for thread-safe token caching
- AgentIdPClient with full method coverage: agents, oauth2, credentials, audit, marketplace, delegation
- Error hierarchy via thiserror (AgentIdPError enum)
- All model types with serde derive
- 429 RateLimited handling with Retry-After parsing; zero unwrap() calls
- Unit tests (mockito), doc tests, and integration tests (#[ignore])
- quickstart example, full README, cargo doc clean

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sdk-rust/Cargo.toml

@@ -0,0 +1,28 @@
+[package]
+name = "sentryagent-idp"
+version = "1.0.0"
+edition = "2021"
+description = "Production-grade Rust SDK for SentryAgent.ai AgentIdP — agent identity, credentials, and A2A delegation"
+authors = ["SentryAgent.ai <sdk@sentryagent.ai>"]
+license = "MIT"
+repository = "https://github.com/sentryagent/sentryagent-idp"
+documentation = "https://docs.rs/sentryagent-idp"
+keywords = ["agent", "identity", "oauth2", "ai", "oidc"]
+categories = ["authentication", "web-programming::http-client"]
+
+[dependencies]
+tokio = { version = "1.35", features = ["full"] }
+reqwest = { version = "0.11", features = ["json", "rustls-tls"], default-features = false }
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
+uuid = { version = "1.6", features = ["v4"] }
+thiserror = "1.0"
+async-trait = "0.1"
+
+[dev-dependencies]
+tokio-test = "0.4"
+mockito = "1.2"
+
+[[example]]
+name = "quickstart"
+path = "examples/quickstart.rs"