Summary

Fixes #1663

• Throw on initial connection failure: SubscribeAsync now catches RpcException from the gRPC stream setup and throws a DaprException with context (topic name, pubsub name), allowing callers to detect and handle sidecar unavailability
• Error callback for runtime errors: Added SubscriptionErrorHandler delegate and ErrorHandler property on DaprSubscriptionOptions so callers can receive DaprException-wrapped errors from background tasks instead of having them become UnobservedTaskExceptions
• Retryable after failure: Resets hasInitialized flag on both initial connection failures and mid-stream background task failures, so callers can retry SubscribeAsync
• Safe error handler invocation: Guards ErrorHandler?.Invoke() with try/catch to prevent a faulty handler from recreating unobserved exceptions
• ContinueWith uses CancellationToken.None: Ensures error-handling continuations always execute even after cancellation

Test plan

• HandleTaskCompletion invokes ErrorHandler with DaprException wrapping the original exception
• HandleTaskCompletion does not throw when no ErrorHandler is set
• HandleTaskCompletion does not throw when ErrorHandler itself throws
• HandleTaskCompletion does not invoke ErrorHandler for successful tasks
• SubscribeAsync throws DaprException wrapping RpcException when sidecar unavailable
• SubscribeAsync allows retry after RpcException failure (hasInitialized reset)
• SubscribeAsync resets hasInitialized for non-RPC exceptions (e.g. ObjectDisposedException)
• All 22 tests pass on net8.0, net9.0, and net10.0
• Tests use xUnit v3 patterns (TestContext.Current.CancellationToken)

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