Disallow expect via lints (#865)

Adds `expect()` as a denied lint. Same deal applies with `unwrap()`
where we now need to put `#[expect(...` on ones that we legit want. Took
care to enable `expect()` in test contexts.

# Tests

```
cargo fmt
cargo clippy --all-features --all-targets --no-deps -- -D warnings
cargo test
```

codex-rs/execpolicy/tests/bad.rs

@@ -1,3 +1,4 @@
+#![expect(clippy::expect_used)]
 use codex_execpolicy::NegativeExamplePassedCheck;
 use codex_execpolicy::get_default_policy;
 

codex-rs/execpolicy/tests/cp.rs

@@ -1,3 +1,4 @@
+#![expect(clippy::expect_used)]
 extern crate codex_execpolicy;
 
 use codex_execpolicy::ArgMatcher;

codex-rs/execpolicy/tests/good.rs

@@ -1,3 +1,4 @@
+#![expect(clippy::expect_used)]
 use codex_execpolicy::PositiveExampleFailedCheck;
 use codex_execpolicy::get_default_policy;
 

codex-rs/execpolicy/tests/head.rs

@@ -1,3 +1,4 @@
+#![expect(clippy::expect_used)]
 use codex_execpolicy::ArgMatcher;
 use codex_execpolicy::ArgType;
 use codex_execpolicy::Error;

codex-rs/execpolicy/tests/literal.rs

@@ -1,3 +1,4 @@
+#![expect(clippy::expect_used)]
 use codex_execpolicy::ArgType;
 use codex_execpolicy::Error;
 use codex_execpolicy::ExecCall;

codex-rs/execpolicy/tests/ls.rs

@@ -1,3 +1,4 @@
+#![expect(clippy::expect_used)]
 extern crate codex_execpolicy;
 
 use codex_execpolicy::ArgType;

codex-rs/execpolicy/tests/pwd.rs

@@ -1,3 +1,4 @@
+#![expect(clippy::expect_used)]
 extern crate codex_execpolicy;
 
 use std::vec;

codex-rs/execpolicy/tests/sed.rs

@@ -1,3 +1,4 @@
+#![expect(clippy::expect_used)]
 extern crate codex_execpolicy;
 
 use codex_execpolicy::ArgType;