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/config/src/requirements_exec_policy.rs

@@ -21,6 +21,8 @@ pub struct RequirementsExecPolicyToml {
 /// This shares the builtin's matching semantics and additionally supports managed `prompt-user` decisions.
 #[derive(Debug, Clone, PartialEq, Eq, Deserialize)]
 pub struct RequirementsExecPolicyPrefixRuleToml {
+    #[serde(default)]
+    pub canonical_matching: bool,
     pub pattern: Vec<RequirementsExecPolicyPatternTokenToml>,
     pub decision: Option<RequirementsExecPolicyDecisionToml>,
     pub justification: Option<String>,
@@ -136,6 +138,7 @@ impl RequirementsExecPolicyToml {
             for head in first_token.alternatives() {
                 let rule: RuleRef = Arc::new(PrefixRule {
                     pattern: PrefixPattern {
+                        canonical_matching: rule.canonical_matching,
                         first: Arc::from(head.as_str()),
                         rest: rest.clone(),
                     },