Add reusable OAuth and auth-source foundations

Add runtime OAuth primitives for PKCE generation, authorization URL building, token exchange request shaping, and refresh request shaping. Wire the API client to a real auth-source abstraction so future OAuth tokens can flow into Anthropic requests without bespoke header code.

This keeps the slice bounded to foundations: no browser flow, callback listener, or token persistence. The API client still behaves compatibly for current API-key users while gaining explicit bearer-token and combined auth modeling.

Constraint: Must keep the slice minimal and real while preserving current API client behavior
Constraint: Repo verification requires fmt, tests, and clippy to pass cleanly
Rejected: Implement full OAuth browser/listener flow now | too broad for the current parity-unblocking slice
Rejected: Keep auth handling as ad hoc env reads only | blocks reuse by future OAuth integration paths
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Extend OAuth behavior by composing these request/auth primitives before adding session or storage orchestration
Tested: cargo fmt --all; cargo clippy -p runtime -p api --all-targets -- -D warnings; cargo test -p runtime; cargo test -p api --tests
Not-tested: live OAuth token exchange; callback listener flow; workspace-wide tests outside runtime/api

rust/crates/api/src/lib.rs

@@ -3,7 +3,7 @@ mod error;
 mod sse;
 mod types;
 
-pub use client::{AnthropicClient, MessageStream};
+pub use client::{AnthropicClient, AuthSource, MessageStream, OAuthTokenSet};
 pub use error::ApiError;
 pub use sse::{parse_frame, SseParser};
 pub use types::{