fix: SubscribeAsync throws DaprException when sidecar is unavailable

[Copilot]

Summary

Fixes #1663 - SubscribeAsync silently fails with unobserved task exceptions when Dapr sidecar is unavailable.

Changes

1. Initial connection failure now throws DaprException

SubscribeAsync catches RpcException from the gRPC stream setup and wraps it in a DaprException with a descriptive message including the topic and pubsub names. Callers get a catchable exception immediately instead of a silent no-op.

2. Retryability

The hasInitialized flag is reset on any failure path during initial connection, so callers can retry SubscribeAsync after the sidecar becomes available without needing to create a new receiver.

3. Background task error handling via SubscriptionErrorHandler

Added SubscriptionErrorHandler delegate and ErrorHandler property on DaprSubscriptionOptions. When background tasks (sidecar streaming, acknowledgement processing, message processing) fault after a successful subscription, HandleTaskCompletion invokes this callback with a DaprException wrapping the original error.

If no handler is configured, the pre-existing behavior is preserved (exception re-thrown → unobserved task exception via default .NET behavior).

4. RpcException wrapped in DaprException

Both failure paths now wrap in DaprException for consistency with the rest of the SDK:

• Initial connection failure: DaprException with message referencing the topic/pubsub name, inner RpcException
• Background task failure: DaprException with message referencing the active subscription, inner exception from the faulted task

Test coverage

All scenarios have dedicated unit tests validating the behavior (65 tests pass).

Breaking changes

• HandleTaskCompletion is now an instance method (was static). This is an internal method so no public API break.
• When no ErrorHandler is configured and a background task faults, the thrown exception type changes from AggregateException to DaprException (wrapping the original exception).

[codecov]

Codecov Report

❌ Patch coverage is 89.28571% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.54%. Comparing base (843b73a) to head (5e41a48).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...aging/PublishSubscribe/PublishSubscribeReceiver.cs	88.88%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1803      +/-   ##
==========================================
+ Coverage   65.50%   65.54%   +0.04%     
==========================================
  Files         295      295              
  Lines        8658     8678      +20     
  Branches     1003     1004       +1     
==========================================
+ Hits         5671     5688      +17     
- Misses       2742     2745       +3     
  Partials      245      245              

Flag	Coverage Δ
net10.0	65.53% <89.28%> (+0.04%)	⬆️

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