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`

Cargo.toml

@@ -6,3 +6,7 @@ members = [
     "mcp-stdio"
 ]
 resolver = "2"
+
+[workspace.lints.rust]
+unused_imports = "deny"
+