[Context]: add Polly resilience adapter for operations

[rian-be]

Summary

Add a Polly resilience adapter so retry/circuit-breaker policies can be attached to context-scoped operations.

Motivation

Batch operations hitting external APIs need resilience policies (retry, timeout, circuit breaker) without re-implementing them per call site.

Example

var policy = Policy.Handle<HttpRequestException>().RetryAsync(3);

await manager.ExecuteWithPolicy(ctx, policy, async () =>
{
    await externalApi.Call();
});

Design Expectations

• Adapter-level integration (depends on Polly); Core stays neutural.
• Policy wraps the context-scoped delegate; context isolation/events unchanged on the delegate.
• Composition with batch primitives (apply a policy to Each item).

Non-Goals

• No reimplementation of retry/circuit logic in the library.
• No defaults — resilience is opt-in.