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.

codex-rs/mcp-client/Cargo.toml

@@ -11,11 +11,11 @@ serde_json = "1"
 tracing = { version = "0.1.41", features = ["log"] }
 tracing-subscriber = { version = "0.3", features = ["fmt", "env-filter"] }
 tokio = { version = "1", features = [
-    "io-std",
+    "io-util",
     "macros",
     "process",
     "rt-multi-thread",
-    "signal",
+    "sync",
 ] }
 
 [dev-dependencies]