Activity.Current fixes

There are some scenarios where directly referencing `System.Diagnostics.DiagnosticSource` can break usage of `Activity.Current`:
- referencing (even transitively) `DiagnosticSource` and deploying a `function.deps.json` file -- which happens by default now.
- directly referencing a version of `DiagnosticSource` that is ahead of the host.

We need tests around these and to investigate a fix that can make this hard to break in the future.

No files, tests, or entry points are named. Start by reproducing the two Activity.Current failure scenarios described in the issue, then add regression coverage for each and investigate a durable host-side fix. Done means both deployment/version cases are covered and Activity.Current remains usable without breaking existing scenarios.

Written by the indexing model from the issue text.