Ensure all jobs complete before disposing parent scope

[mikaelweave]

Description

Changes JobHosting to wait for all jobs to complete before starting disposal. We have been getting ObjectDisposedExceptions after the HostingBackgroundService exist. This tells me that the background service is exiting before all jobs are done.

This PR changes the JobHosting error logic to request cancellation before returning, thus delaying the HostingBackgroundService exit until jobs are complete.

Related issues

AB#121830

FHIR Team Checklist

• Update the title of the PR to be succinct and less than 65 characters
• Add a milestone to the PR for the sprint that it is merged (i.e. add S47)
• Tag the PR with the type of update: Bug, Build, Dependencies, Enhancement, New-Feature or Documentation
• Tag the PR with Open source, Azure API for FHIR (CosmosDB or common code) or Azure Healthcare APIs (SQL or common code) to specify where this change is intended to be released.
• Tag the PR with Schema Version backward compatible or Schema Version backward incompatible or Schema Version unchanged if this adds or updates Sql script which is/is not backward compatible with the code.
• CI is green before merge
• Review squash-merge requirements

Semver Change (docs)

Patch|Skip|Feature|Breaking (reason)

[azure-boards]

✅ Successfully linked to Azure Boards work item(s):

• Bug 121830