[C#] Deduplicate OnDisposeCall and improve implementation

[Copilot]

• Explore both ActionDisposable in Client.cs and OnDisposeCall in Session.cs
• Verify build/test baseline
• Create ActionDisposable.cs as a standalone file with the better implementation from Client.cs
• Remove ActionDisposable class definition from Client.cs
• Replace OnDisposeCall usage in Session.cs with ActionDisposable
• Build and test — build passes; all test failures are pre-existing (missing tsx in sandbox)
• Code review and security scan — no issues found

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[Copilot]

Pull request overview

This PR deduplicates disposable-helper implementations in the .NET SDK by centralizing the “run an action on dispose” pattern into a single reusable type, and switching Session’s event unsubscription to use it.

Changes:

• Introduced a standalone ActionDisposable helper with a thread-safe, idempotent Dispose() implementation.
• Removed the duplicated ActionDisposable implementation from Client.cs.
• Replaced Session’s private OnDisposeCall with ActionDisposable for handler unsubscription.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
dotnet/src/Session.cs	Uses ActionDisposable to unsubscribe event handlers and removes the now-redundant private disposable type.
dotnet/src/Client.cs	Removes the inline ActionDisposable definition now that it’s centralized.
dotnet/src/ActionDisposable.cs	Adds the shared ActionDisposable implementation for use across the .NET SDK.