Add namespace and improve ProgressToken null handling

[Kamyab7]

Added namespace declaration for EverythingServer and improved ProgressToken null handling to enhance code organization and prevent potential null reference issues.

Motivation and Context

1. The SubscriptionMessageSender class was missing a namespace declaration, which could lead to organization and accessibility issues.
2. The ProgressToken.ToString() method was returning null in certain cases, which could cause null reference exceptions in consuming code. This change ensures consistent behavior by returning an empty string instead.

How Has This Been Tested?

• Verified that the namespace addition resolves compilation issues in the EverythingServer project
• Tested ProgressToken.ToString() with various input types (string, long, and null) to ensure proper string conversion
• Confirmed that existing functionality remains intact while improving null safety

Breaking Changes

No breaking changes. The modifications are backward compatible:

• Namespace addition only affects internal organization
• Empty string return instead of null is a safer alternative that maintains the same contract

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

The changes focus on improving code quality and safety:

1. The namespace addition follows C# conventions for code organization
2. The empty string return in ProgressToken.ToString() follows the principle of least surprise and prevents null reference exceptions
3. Both changes are minimal and focused, making them easy to review and maintain

[stephentoub]

which could cause null reference exceptions in consuming code

In what code?

[Kamyab7]

which could cause null reference exceptions in consuming code

In what code?

I haven't found any actual issues in the current codebase, but I was being proactive about preventing potential null reference exceptions if someone misuses the method in the future. It's more of a defensive programming approach rather than fixing an existing bug.