Reference System.Net.ServerSentEvents only on net8.0/net9.0

[glen-84]

Summary

• System.Net.ServerSentEvents (the SseParser API) was introduced as a standalone package in .NET 9 but is in-box in the net10.0 shared framework. Referencing it unconditionally tripped NU1510 ("will not be pruned … likely unnecessary"), which broke the Nitro CLI AOT publish in the Release workflow — that path forces a single-targeted net10.0 restore (-p:TargetFrameworks=net10.0) under TreatWarningsAsErrors.
• PR CI never caught it: it restores multi-TFM (net8.0;net9.0;net10.0), where the package is genuinely required for the lower TFMs, so NU1510 is not emitted; and it doesn't build the Nitro CLI binary at all.
• Gate the package reference to net8.0/net9.0 in both consumers (Transport.Http, Fusion.Execution); on net10.0 the in-box type is used.

Test plan

• Single-TFM net10.0 build of both projects now succeeds — no NU1510, and compiles against the in-box type.
• net8.0/net9.0 builds still resolve and use the package.
• Per-TFM restore matrix clean for both projects across net8.0, net9.0, and net10.0 (previously net10.0 → NU1510).

[Copilot]

Pull request overview

This PR fixes a restore/AOT publish failure by only referencing System.Net.ServerSentEvents when targeting frameworks that need the standalone package (net8.0/net9.0), avoiding NU1510 on net10.0 where the API is in-box.

Changes:

• Gate System.Net.ServerSentEvents PackageReference to net8.0 and net9.0 in Fusion.Execution.
• Gate System.Net.ServerSentEvents PackageReference to net8.0 and net9.0 in Transport.Http.

Reviewed changes

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

File	Description
src/HotChocolate/Fusion/src/Fusion.Execution/HotChocolate.Fusion.Execution.csproj	Adds TFM-conditional package reference so net10.0 avoids NU1510 while net8.0/net9.0 still restore the package.
src/HotChocolate/AspNetCore/src/Transport.Http/HotChocolate.Transport.Http.csproj	Same TFM-conditional package reference to keep SSE support on net8.0/net9.0 without pulling the package on net10.0.

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[github-code-quality]

Code Coverage Overview

Languages: C#

C# / code-coverage/dotnet

The overall coverage in the gai/serversentevents... branch remains at 49%, unchanged from the main branch.

Show a code coverage summary of the most impacted files.

File	main 4691dfa	gai/serversentevents... d6628c8	+/-
/home/runner/wo...tDiagnostics.cs	88%	73%	-15%
/home/runner/wo...entsListener.cs	66%	62%	-4%
/home/runner/wo...gerMessage.g.cs	19%	15%	-4%
/home/runner/wo...PromiseCache.cs	81%	78%	-3%
/home/runner/wo...geDispatcher.cs	77%	75%	-2%
/home/runner/wo...DefaultTopic.cs	90%	89%	-1%
/home/runner/wo...qlConnection.cs	89%	92%	+3%
/home/runner/wo...gerMessage.g.cs	6%	10%	+4%
/home/runner/wo...eiveEndpoint.cs	72%	77%	+5%
/home/runner/wo...gerMessage.g.cs	15%	27%	+12%

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