chore: strictly deny unused imports workspace-wide

- Added `unused_imports = "deny"` to workspace lints in Cargo.toml
- Opted-in all crates to inherit workspace lints
- Fixed resulting unused imports in `dump_schema.rs`

stub/Cargo.toml

@@ -20,3 +20,6 @@ async-channel = "2.5.0"
 
 [build-dependencies]
 chrono = "0.4.45"
+
+[lints]
+workspace = true