[release/7.0-rc1] Omit the JS sync context if threads are not enabled

[github-actions]

Backport of #74167 to release/7.0-rc1

/cc @maraf@kg

Customer Impact

Testing

Risk

IMPORTANT: Is this backport for a servicing release? If so and this change touches code that ships in a NuGet package, please make certain that you have added any necessary package authoring and gotten it explicitly reviewed.

[ghost]

Tagging subscribers to 'arch-wasm': @lewing
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Issue Details

---

Backport of #74167 to release/7.0-rc1

/cc @maraf@kg

Customer Impact

Testing

Risk

IMPORTANT: Is this backport for a servicing release? If so and this change touches code that ships in a NuGet package, please make certain that you have added any necessary package authoring and gotten it explicitly reviewed.

Author:	github-actions[bot]
Assignees:	-
Labels:	arch-wasm, area-System.Runtime.InteropServices.JavaScript
Milestone:	-

[kg]

The failure log for Linux x64 Debug doesn't actually show any test failures:

 Finished: System.Net.Http.Functional.Tests
=== TEST EXECUTION SUMMARY ===
System.Net.Http.Functional.Tests Total: 3776, Errors: 0, Failed: 0, Skipped: 14, Time: 120.634s

[carlossanlop]

@marek-safar@lewing need an approval before I merge.

[lewing]

This fixes a size regression.

This will break blazor's very fragile manifest checks in the sdk but they already need to be updated (see dotnet/sdk#27307)

cc @TanayParikh

[carlossanlop]

This was approved, signed-off and the CI failure is unrelated. Ready to merge.