Fix Fusion keyed-service cast build break on older TFMs

[glen-84]

Summary

• The keyed-service factory key is non-nullable object on net11.0 but object? on net8.0-net10.0, so casting it to string needs a null-forgiving suppression on the older TFMs.
• The IDE0370 cleanup in Enable IDE0370 and remove unnecessary suppression operators #10012 was computed against net11.0 and stripped that suppression, breaking the net8.0/net9.0/net10.0 builds with CS8600 (promoted to an error via WarningsAsErrors;nullable). CI's per-area test jobs build only net11.0, so the break was silent.
• Guard the cast behind #if NET11_0_OR_GREATER so net11.0 stays suppression-free (no IDE0370) and the older TFMs keep the required !.

Test plan

• dotnet build of HotChocolate.Fusion.Execution succeeds on net11.0, net10.0, net9.0, and net8.0.

[Copilot]

Pull request overview

Fixes a multi-targeting build break in Fusion’s DI setup caused by differing nullability annotations for keyed-service factory keys across TFMs (net8–net10 vs net11+).

Changes:

• Add TFM-conditional compilation around the keyed-service key cast to keep net11+ suppression-free while restoring the needed null-forgiving operator on older TFMs.

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[github-actions]

Patch coverage

100.0% of changed lines covered (6/6)

File	Covered	Changed	Patch %
…/HotChocolateFusionServiceCollectionExtensions.cs	6	6	100.0% 🟢

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Project coverage: 52.4% (216289/412383 lines)