ManagedWebSocket: report actual messageType in invalid-type ArgumentException

[Copilot]

ManagedWebSocket.ThrowIfInvalidMessageType hardcoded "Close" into the ArgumentException message regardless of the value supplied by the caller, so e.g. SendAsync(..., (WebSocketMessageType)999, ...) produced a misleading "The message type 'Close' is not allowed...".

• Pass the actual messageType through to the SR.Format call (placeholder {0}) in src/libraries/System.Net.WebSockets/src/System/Net/WebSockets/ManagedWebSocket.cs, replacing the hardcoded nameof(WebSocketMessageType.Close).
• Mirrors what BrowserWebSocket.SendAsync already does, so behavior is now consistent across implementations and future-proof against new WebSocketMessageType values.

// Before: always "Close"
// After: "999" for (WebSocketMessageType)999, "Close" for WebSocketMessageType.Close, etc.
static void ThrowInvalidMessageType(WebSocketMessageType messageType, string? paramName) =>
    throw new ArgumentException(SR.Format(
        SR.net_WebSockets_Argument_InvalidMessageType,
        messageType, nameof(SendAsync), nameof(WebSocketMessageType.Binary),
        nameof(WebSocketMessageType.Text), nameof(CloseOutputAsync)),
        paramName);

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[Copilot]

Pull request overview

Updates ManagedWebSocket.ThrowIfInvalidMessageType so the ArgumentException message reports the actual invalid WebSocketMessageType value provided by the caller (including undefined enum values), rather than always reporting "Close".

Changes:

• Passes the messageType argument through to the SR.Format placeholder for the invalid-message-type exception.
• Updates the local throw helper signature to accept messageType and propagate it into the formatted message.