Add opt-in Unicode canonical matching to exec policy rules (#59535)

## What changed

- Support `canonical_matching=True` in `prefix_rule` and `canonical_matching = true` in managed TOML prefix rules. Apply Unicode canonical caseless matching to every prefix token, including alternatives, while keeping exact matching as the default.
- Match case and canonically equivalent Unicode spellings, such as `bash` and `bAſh`, without resolving executables on the filesystem.
- Preserve managed restrictions when checking absolute-path rule amendments, and enforce applicable `host_executable` path allowlists during basename fallback.

## Testing

Add coverage for case folding, Unicode normalization, token alternatives, exact-match defaults, and host executable allowlists. Extend managed-policy and shell-execution tests to cover canonical matching.

GitOrigin-RevId: c2f79069dc38d55c77028c457b22beb468d5c571

codex-rs/execpolicy/src/parser.rs

@@ -352,6 +352,7 @@ fn policy_builtins(builder: &mut GlobalsBuilder) {
         r#match: Option<UnpackList<Value<'v>>>,
         not_match: Option<UnpackList<Value<'v>>>,
         justification: Option<&'v str>,
+        canonical_matching: Option<bool>,
         eval: &mut Evaluator<'v, '_, '_>,
     ) -> anyhow::Result<NoneType> {
         let decision = match decision {
@@ -393,6 +394,7 @@ fn policy_builtins(builder: &mut GlobalsBuilder) {
             .map(|head| {
                 Arc::new(PrefixRule {
                     pattern: PrefixPattern {
+                        canonical_matching: canonical_matching.unwrap_or(false),
                         first: Arc::from(head.as_str()),
                         rest: rest.clone(),
                     },