Fix to allow reuse of group names when using Redis/NATS as backplane

[mayuki]

No description provided.

[Copilot]

Pull Request Overview

This PR fixes an issue that prevented reusing group names with Redis and NATS backplanes by ensuring that disposed groups are properly removed from the internal dictionaries. Key changes include:

• Adding tests (for in-memory, Redis, and NATS) to verify that groups can be recreated after disposal.
• Updating the provider classes to pass a removal callback to group implementations.
• Modifying group classes (RedisGroup and NatsGroup) to invoke a disposal callback to remove disposed groups.

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/Multicaster.Tests/InMemoryGroupTest.cs	Added a test to verify group reuse after disposal in the in-memory provider.
test/Multicaster.Distributed.Redis.Tests/RedisGroupTest.cs	Updated tests to use the helper CreateJsonSerializedInvocation for assertions.
test/Multicaster.Distributed.Nats.Tests/NatsGroupTest.cs	Added a test to verify group reuse after disposal with the NATS provider.
src/Multicaster.Distributed.Redis/RedisGroupProvider.cs	Updated group creation methods to pass a removal callback that cleans up disposed groups.
src/Multicaster.Distributed.Redis/RedisGroup.cs	Modified Dispose to call the removal callback for group cleanup.
src/Multicaster.Distributed.Nats/NatsGroupProvider.cs	Updated group creation methods to pass a removal callback to NatsGroup.
src/Multicaster.Distributed.Nats/NatsGroup.cs	Modified Dispose to call the removal callback for group cleanup.