routePrefix cannot be overridden by AzureFunctionsJobHost__extensions__http__routePrefix environment variable

[MatthewFisher-eaton]

Description

routePrefix is currently read directly from the host.json file with no provision to override from an environment variable.

See FunctionsEndpointDataSource.BuildEndpoints.

The host allows the override via AzureFunctionsJobHost__extensions__http__routePrefix, but then the call to the worker fails with a timeout because the routePrefix does not match.

Steps to reproduce

Minimal repro attached as RoutePrefixOverride.zip

• Create a dotnet-isolated function called Version with a route prefix of api/foo in host.json.
• Verify api/foo/version responds as expected.
• Add the following in local.settings.json:

     "AzureFunctionsJobHost__extensions__http__routePrefix": "api/bar"

• Restart the function.
• Confirm the original api/foo/version now returns a 404.
• Confirm the new api/bar/version returns a 500.

[MatthewFisher-eaton]

This was discovered while migrating a Function from in-process to an isolated-worker.