Auto-populate completion handlers from AllowedValuesAttribute on prompt/resource parameters

[Copilot]

• Remove intermediate array in completion value filtering (use direct foreach with StartsWith check)
• Guard against non-object properties JSON value before enumerating
• Use is instead of == for JsonValueKind comparison
• Guard against non-string enum array items using ValueKind is JsonValueKind.String check
• Update XML docs on McpServerPromptAttribute, McpServerResourceAttribute, McpServerPrompt, and McpServerResource to note that string parameters with AllowedValuesAttribute automatically surface completions
• Guard AllowedValues test methods with #if NET for net472 compatibility

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[stephentoub]

@jeffhandley / @mikekistler / @halter73 / @eiriktsarpalis , thoughts on using [AllowedValues] for this?

[stephentoub]

@copilot there are build failures for net472; the use of AllowedValues needs to be guarded to just #if NET

[Copilot]

@copilot there are build failures for net472; the use of AllowedValues needs to be guarded to just #if NET

Wrapped all 5 AllowedValues test methods with #if NET / #endif in 90dd295. The AllowedValuesAttribute type isn't available on net472, so those tests are now excluded from that TFM.