Move options validation to constructor

[halter73]

This works around the breaking change to BackgroundService in dotnet/runtime#116283 which launches ExecuteAsync using Task.Run.

Fixes #823

[Copilot]

Pull Request Overview

This PR moves options validation from the ExecuteAsync method to the constructor in IdleTrackingBackgroundService to address a breaking change in .NET 10 where BackgroundService launches ExecuteAsync using Task.Run, potentially causing validation exceptions to be lost.

• Refactored IdleTrackingBackgroundService to use traditional constructor with field assignments instead of primary constructor
• Moved validation logic from ExecuteAsync to constructor to ensure early failure
• Removed conditional compilation directives that were skipping tests on .NET 10

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
IdleTrackingBackgroundService.cs	Converted from primary constructor to traditional constructor with field assignments and moved validation logic
StreamableHttpServerConformanceTests.cs	Removed .NET 10 conditional compilation that was skipping validation tests

Comments suppressed due to low confidence (1)

src/ModelContextProtocol.AspNetCore/IdleTrackingBackgroundService.cs:1

• The validation logic should validate the options before assigning them to fields. Move these validation calls to occur before the field assignments at lines 28-31 to fail fast if invalid options are provided.

using Microsoft.Extensions.Hosting;

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