fix: build all crates individually as part of CI (#833)

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.

.github/workflows/rust-ci.yml

@@ -83,6 +83,15 @@ jobs:
       - name: cargo clippy
         run: cargo clippy --target ${{ matrix.target }} --all-features -- -D warnings || echo "FAILED=${FAILED:+$FAILED, }cargo clippy" >> $GITHUB_ENV
 
+      # Running `cargo build` from the workspace root builds the workspace using
+      # the union of all features from third-party crates. This can mask errors
+      # where individual crates have underspecified features. To avoid this, we
+      # run `cargo build` for each crate individually, though because this is
+      # slower, we only do this for the x86_64-unknown-linux-gnu target.
+      - name: cargo build individual crates
+        if: ${{ matrix.target == 'x86_64-unknown-linux-gnu' }}
+        run: find . -name Cargo.toml -mindepth 2 -maxdepth 2 -print0 | xargs -0 -n1 -I{} bash -c 'cd "$(dirname "{}")" && cargo build' || echo "FAILED=${FAILED:+$FAILED, }cargo build individual crates" >> $GITHUB_ENV
+
       - name: cargo test
         run: cargo test --target ${{ matrix.target }} || echo "FAILED=${FAILED:+$FAILED, }cargo test" >> $GITHUB_ENV