Make Tool.Name required to match other protocol types

[Copilot]

Tool.Name used string Name { get; set; } = string.Empty while Prompt.Name, Resource.Name, ResourceTemplate.Name, and PromptArgument.Name all use required string Name. This allowed constructing a Tool with an empty-string name without a compiler warning.

• Changed Tool.Name to required string Name { get; set; } to match all other IBaseMetadata implementations
• Updated three test sites in ToolTests.cs that constructed Tool without setting Name

Breaking change: callers must now specify Name in object initializers.

// Before: compiles silently with empty name
var tool = new Tool();

// After: compiler error CS9035, Name is required
var tool = new Tool { Name = "my_tool" };

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