feat(storage): add IsOpen API for zonal read operation

[v-pratap]

Add IsOpen() to ObjectDescriptor to allow applications to detect if a descriptor is still valid without hanging on a dead connection.

The IsOpen() method returns false if the descriptor has been explicitly cancelled or if all underlying gRPC streams have permanently failed and cannot be resumed. It also incorporates a health check of the underlying gRPC channels.

[gemini-code-assist]

Code Review

This pull request introduces an IsOpen() method to the ObjectDescriptor class and its associated connection interfaces, allowing users to verify if a descriptor is still active and the underlying transport is healthy. The implementation includes gRPC channel state monitoring and updated unit tests. Review feedback highlighted a potential crash in ObjectDescriptorImpl::IsOpen() if the transport callback is null, as well as the risk of calling this callback while holding a mutex. Additionally, the permanent_failure_ field was identified as unused and should be either implemented or removed.

[codecov]

Codecov Report

❌ Patch coverage is 89.90826% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.69%. Comparing base (fdbec33) to head (57b8c28).

Files with missing lines	Patch %	Lines
...le/cloud/storage/internal/async/connection_impl.cc	57.14%	9 Missing ⚠️
...cloud/storage/async/object_descriptor_connection.h	0.00%	1 Missing ⚠️
...rnal/async/object_descriptor_connection_tracing.cc	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16063      +/-   ##
==========================================
- Coverage   92.70%   92.69%   -0.01%     
==========================================
  Files        2343     2343              
  Lines      216644   216740      +96     
==========================================
+ Hits       200838   200910      +72     
- Misses      15806    15830      +24     

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