Fix gRPC channel cache key to include all channel-affecting properties

[Copilot]

The gRPC channel cache key only included optionsName, EndpointAddress, and TaskHubName, causing stale channels to be reused when other channel-affecting properties changed (e.g., via options reload).

Changes

• Worker extensions: Cache key now includes ResourceId, credential type, AllowInsecureCredentials, and WorkerId
• Client extensions: Cache key now includes ResourceId, credential type, AllowInsecureCredentials, and RetryOptions
• Delimiter: Changed from : to \u001F (Unicode unit separator) to avoid collisions with endpoint URIs
• Tests: Added 8 new tests verifying separate channels for distinct configurations; fixed 4 existing tests to use async disposal

Impact

Before this fix, changing any of these properties while keeping the same endpoint/task hub would incorrectly reuse the cached channel:

// These would incorrectly share a channel despite different ResourceIds
builder.UseDurableTaskScheduler(endpoint, hub, cred, opts => opts.ResourceId = "https://durabletask.io");
builder.UseDurableTaskScheduler(endpoint, hub, cred, opts => opts.ResourceId = "https://custom.io");

Now each unique configuration creates its own channel as intended.

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