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`

mcp-stdio/Cargo.toml

@@ -8,3 +8,6 @@ tokio = { version = "1.53.1", features = ["io-util", "macros"] }
 
 [dev-dependencies]
 tokio = { version = "1.53.1", features = ["rt", "macros"] }
+
+[lints]
+workspace = true