fix(core): respect reject policy by approval source for skill scripts (#13816)

## Summary
- distinguish reject-policy handling for prefix-rule approvals versus
sandbox approvals in Unix shell escalation
- keep prompting for skill-script execution when `rules=true` but
`sandbox_approval=false`, instead of denying the command up front
- add regression coverage for both skill-script reject-policy paths in
`codex-rs/core/tests/suite/skill_approval.rs`

codex-rs/core/src/exec_policy.rs

@@ -104,7 +104,7 @@ fn is_policy_match(rule_match: &RuleMatch) -> bool {
 /// `prompt_is_rule` distinguishes policy-rule prompts from sandbox/escalation
 /// prompts so `Reject.rules` and `Reject.sandbox_approval` are honored
 /// independently. When both are present, policy-rule prompts take precedence.
-fn prompt_is_rejected_by_policy(
+pub(crate) fn prompt_is_rejected_by_policy(
     approval_policy: AskForApproval,
     prompt_is_rule: bool,
 ) -> Option<&'static str> {