Add IAsyncDisposable to CosmosService and fix async disposal pattern

[anuchandy]

What does this PR do?

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The .NET DI container prefers DisposeAsync() during shutdown, pr Implement IAsyncDisposable in CosmosService. Following https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/implementing-disposeasync

Fixes: #1148

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GitHub issue number?

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Pre-merge Checklist

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[Copilot]

Pull request overview

This PR implements IAsyncDisposable in CosmosService to fix an anti-pattern where the Dispose() method was implemented as async void, which prevented callers from awaiting the disposal operation. The implementation follows the Microsoft async disposal pattern guidelines for sealed classes.

Changes:

• Added IAsyncDisposable to the ICosmosService interface and CosmosService class
• Replaced the async void Dispose(bool disposing) method with a proper DisposeAsyncCore() implementation
• Added DisposeAsync() method that properly implements the async disposal pattern
• Modified synchronous Dispose() to call the async core method synchronously

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
tools/Azure.Mcp.Tools.Cosmos/src/Services/ICosmosService.cs	Added IAsyncDisposable interface to ICosmosService
tools/Azure.Mcp.Tools.Cosmos/src/Services/CosmosService.cs	Implemented async disposal pattern with DisposeAsyncCore(), DisposeAsync(), and updated Dispose() methods; marked class as sealed
servers/Azure.Mcp.Server/changelog-entries/1768937539777.yaml	Added changelog entry documenting the bug fix