Use project namespace instead of Microsoft.* namespace for ConfigurationMissingException

[coderabbitai]

Description

The ConfigurationMissingException class in src/Hosting/Exceptions/ConfigurationMissingException.cs currently uses the Microsoft namespace:

namespace Microsoft.Extensions.Hosting.Exceptions;

This should be changed to use the project's namespace instead to avoid confusion and potential conflicts with Microsoft types.

Expected Change

Change the namespace declaration to:

namespace Escendit.Extensions.Hosting.Exceptions;

Additional Context

• Identified during code review of PR feat: add NATS Orleans streaming extensions #15
• Original comment: feat: add NATS Orleans streaming extensions #15 (comment)
• PR: feat: add NATS Orleans streaming extensions #15

Files to Update

• src/Hosting/Exceptions/ConfigurationMissingException.cs - update namespace declaration
• src/Hosting/PublicAPI.Unshipped.txt - update public API entries to reflect new namespace
• Any other files that reference this exception type