feat(server): support external description sources on McpServerTool attribute

[MukundaKatta]

Closes #1516.

Why

[McpServerTool(Description = "...")] only accepts a compiled string literal today, so updating tool guidance for an LLM requires rebuilding and redeploying the server. The issue asks for a way to load tool descriptions from an external source (resx, config, accessor) so they can change without a code release.

What

Adds two opt-in properties to McpServerToolAttribute, mirroring System.ComponentModel.DataAnnotations.DisplayAttribute:

• DescriptionResourceType (Type)
• DescriptionResourceName (string)

When both are set, AIFunctionMcpServerTool.DeriveOptions resolves the description by reflecting against the resource type, in this order:

1. public static property named <Name> returning string
2. public static field named <Name> of type string
3. public static parameterless method named <Name> returning string
4. public static ResourceManager property named ResourceManager (looked up by <Name>)

Resolved value wins over [Description]; if the lookup fails (e.g. missing resource), the existing [Description] fallback still applies. No behavior change when the new properties are unset.

[McpServerTool(
    DescriptionResourceType = typeof(ToolDescriptions),
    DescriptionResourceName = nameof(ToolDescriptions.WeatherSummary))]
public string GetWeather(string city) => ...;

ToolDescriptions.WeatherSummary can be a static property backed by config, a .resx accessor, or any string source the host owns.

Tested

Added tests/ModelContextProtocol.Tests/Server/McpServerToolExternalDescriptionTests.cs with 7 cases covering:

• static property / field / method / ResourceManager resolution
• precedence over [Description]
• fallback to [Description] when the resource is missing
• no-op when only DescriptionResourceName is set without a type