Fix StdioServerTransportTests to use Debug logging level instead of Trace

[Copilot]

Addresses unresolved feedback from #1191: the test class was overriding LoggerFactory to use LogLevel.Trace for all tests, but Debug should remain the default since Trace is too verbose for general test output.

Changes

• Remove class-level LoggerFactory override from constructor (now uses base class default of Debug)
• Update SendMessageAsync_Should_Log_At_Trace_Level and ReadMessagesAsync_Should_Log_Received_At_Trace_Level to create local trace-level logger factories for their specific needs

// Each trace-level test now creates its own logger infrastructure
var mockLoggerProvider = new MockLoggerProvider();
using var traceLoggerFactory = LoggerFactory.Create(builder =>
{
    builder.AddProvider(XunitLoggerProvider);
    builder.AddProvider(mockLoggerProvider);
    builder.SetMinimumLevel(LogLevel.Trace);
});

Original prompt

Address this comment that wasn't handled before the PR was merged: @modelcontextprotocol/csharp-sdk/pull/1191

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