[codex] Fix HTTP workflow and activity fallback guards

[sfmskywalker]

Summary

• move the Elsa base-path gate ahead of route matching so non-Elsa requests skip route resolution
• always restore HttpContext.RequestAborted in the timeout wrapper, including exception and cancellation paths
• fall back to the in-memory workflow state when fault handling cannot reload the workflow instance
• replace the NotFoundActivity null-forgiving fallback with an explicit guard and clear deserialization error

Root Cause

• the middleware resolved routes before confirming the request was even under the Elsa base path
• the timeout helper restored RequestAborted only on the success path
• fault handling assumed workflow-instance reload would always succeed
• activity deserialization assumed the NotFoundActivity descriptor was always registered

Validation

• dotnet test test/unit/Elsa.Http.UnitTests/Elsa.Http.UnitTests.csproj --filter FullyQualifiedName~HttpWorkflowsMiddlewareTests
• dotnet test test/unit/Elsa.Workflows.Core.UnitTests/Elsa.Workflows.Core.UnitTests.csproj --filter FullyQualifiedName~ActivityJsonConverterTests /p:CollectCoverage=false