[FEATURE]: Typestate wrappers for A2A runtime trust-boundary types

[jonpspri]

Context

Surfaced during the PR #3704 review pass on the new Rust A2A runtime. The
SessionRecord.auth_context field (and its downstream consumers in the
trust-header path) is typed as serde_json::Value, which makes it impossible
to encode invariants at the type level — any caller can pull the
auth_context out of a SessionRecord and encode it into a trust header
without having first called validate_fingerprint.

Current code is defense-in-depth correct: in server.rs::handle_a2a_invoke,
fingerprint validation is always called before auth_context is consumed.
But nothing on the type signature of SessionRecord::auth_context enforces
this — a future refactor could drop the check and compile cleanly.

Scope

Introduce typestate wrappers so the compiler rejects trust-boundary misuse:

• struct Authenticated<T>(T) — marker that a Python /authenticate
  round-trip has completed.
• struct FingerprintValidated<T>(T) — marker that
  validate_fingerprint returned true.
• SessionRecord internals shift from raw serde_json::Value for
  auth_context to a newtype (AuthContext) that can only be consumed via
  methods returning Authenticated<&serde_json::Value> or
  FingerprintValidated<&serde_json::Value>.
• trust::encode_auth_context takes Authenticated<&serde_json::Value> (or
  a stricter variant) rather than a bare &serde_json::Value.

Files touched by this refactor

• crates/a2a_runtime/src/session.rs — SessionRecord, SessionManager::lookup, validate_fingerprint
• crates/a2a_runtime/src/trust.rs — encode_auth_context, authorize, authenticate
• crates/a2a_runtime/src/server.rs — call sites in handle_a2a_invoke,
  proxy_task_method, proxy_push_method, proxy_agent_card,
  resolve_agent

Why deferred from PR #3704

This is a design-level refactor with a wide blast radius. Every call site
that threads auth_context through the trust layer needs updating, and the
API of SessionRecord changes visibly. Better handled as its own
coordinated change with migration notes rather than mixed into the PR that
introduced the Rust runtime.

Acceptance criteria

• Compile-time rejection of code paths that consume auth_context
  before fingerprint validation.
• All existing tests in crates/a2a_runtime/ continue to pass.
• A new test demonstrates that a naive misuse (consuming
  auth_context without validate_fingerprint) fails to compile
  (can be verified via a compile_fail doctest).
• No behavior change observable from the Python side.

References

• Finding surfaced by type-design-analyzer during PR feat: add Rust A2A 1.0 runtime support #3704 review
• Related files: crates/a2a_runtime/src/session.rs:29, crates/a2a_runtime/src/trust.rs:83