chore: enable await-holding clippy lints (#18698)

Follow-up to https://github.com/openai/codex/pull/18178, where we said
the await-holding clippy rule would be enabled separately.

Enable `await_holding_lock` and `await_holding_invalid_type` after the
preceding commits fixed or explicitly documented the current offenders.

codex-rs/Cargo.toml

@@ -383,6 +383,8 @@ zeroize = "1.8.2"
 rust = {}
 
 [workspace.lints.clippy]
+await_holding_invalid_type = "deny"
+await_holding_lock = "deny"
 expect_used = "deny"
 identity_op = "deny"
 manual_clamp = "deny"