I discovered that `cargo build` worked for the entire workspace, but not
for the `mcp-client` or `core` crates.
* `mcp-client` failed to build because it underspecified the set of
features it needed from `tokio`.
* `core` failed to build because it was using a "feature" of its own
crate in the default, no-feature version.
This PR fixes the builds and adds a check in CI to defend against this
sort of thing going forward.
49d040215a
