Allow global network allowlist wildcard (#15549)

## Problem

Today `codex-network-proxy` rejects a global `*` in
`network.allowed_domains`, so there is no static way to configure a
denylist-only posture for public hosts. Users have to enumerate broad
allowlist patterns instead.

## Approach

- Make global wildcard acceptance field-specific: `allowed_domains` can
use `*`, while `denied_domains` still rejects a global wildcard.
- Keep the existing evaluation order, so explicit denies still win first
and local/private protections still apply unless separately enabled.
- Add coverage for the denylist-only behavior and update the README to
document it.

## Validation

- `just fmt`
- `cargo test -p codex-network-proxy` (full run had one unrelated flaky
telemetry test:
`network_policy::tests::emit_block_decision_audit_event_emits_non_domain_event`;
reran in isolation and it passed)
- `cargo test -p codex-network-proxy
network_policy::tests::emit_block_decision_audit_event_emits_non_domain_event
-- --exact --nocapture`
- `just fix -p codex-network-proxy`
- `just argument-comment-lint`

codex-rs/network-proxy/src/policy.rs

@@ -2,6 +2,7 @@
 use crate::config::NetworkMode;
 use anyhow::Context;
 use anyhow::Result;
+use anyhow::bail;
 use anyhow::ensure;
 use globset::GlobBuilder;
 use globset::GlobSet;
@@ -151,19 +152,38 @@ pub(crate) fn is_global_wildcard_domain_pattern(pattern: &str) -> bool {
         .any(|candidate| candidate == "*")
 }
 
-pub(crate) fn compile_globset(patterns: &[String]) -> Result<GlobSet> {
+#[derive(Clone, Copy, PartialEq, Eq)]
+enum GlobalWildcard {
+    Allow,
+    Reject,
+}
+
+pub(crate) fn compile_allowlist_globset(patterns: &[String]) -> Result<GlobSet> {
+    compile_globset_with_policy(patterns, GlobalWildcard::Allow)
+}
+
+pub(crate) fn compile_denylist_globset(patterns: &[String]) -> Result<GlobSet> {
+    compile_globset_with_policy(patterns, GlobalWildcard::Reject)
+}
+
+fn compile_globset_with_policy(
+    patterns: &[String],
+    global_wildcard: GlobalWildcard,
+) -> Result<GlobSet> {
     let mut builder = GlobSetBuilder::new();
     let mut seen = HashSet::new();
     for pattern in patterns {
-        ensure!(
-            !is_global_wildcard_domain_pattern(pattern),
-            "unsupported global wildcard domain pattern \"*\"; use exact hosts or scoped wildcards like *.example.com or **.example.com"
-        );
+        if global_wildcard == GlobalWildcard::Reject && is_global_wildcard_domain_pattern(pattern) {
+            bail!(
+                "unsupported global wildcard domain pattern \"*\"; use exact hosts or scoped wildcards like *.example.com or **.example.com"
+            );
+        }
         let pattern = normalize_pattern(pattern);
         // Supported domain patterns:
         // - "example.com": match the exact host
         // - "*.example.com": match any subdomain (not the apex)
         // - "**.example.com": match the apex and any subdomain
+        // - "*": match every host when explicitly enabled for allowlist compilation
         for candidate in expand_domain_pattern(&pattern) {
             if !seen.insert(candidate.clone()) {
                 continue;
@@ -333,7 +353,7 @@ mod tests {
 
     #[test]
     fn compile_globset_normalizes_trailing_dots() {
-        let set = compile_globset(&["Example.COM.".to_string()]).unwrap();
+        let set = compile_denylist_globset(&["Example.COM.".to_string()]).unwrap();
 
         assert_eq!(true, set.is_match("example.com"));
         assert_eq!(false, set.is_match("api.example.com"));
@@ -341,7 +361,7 @@ mod tests {
 
     #[test]
     fn compile_globset_normalizes_wildcards() {
-        let set = compile_globset(&["*.Example.COM.".to_string()]).unwrap();
+        let set = compile_denylist_globset(&["*.Example.COM.".to_string()]).unwrap();
 
         assert_eq!(true, set.is_match("api.example.com"));
         assert_eq!(false, set.is_match("example.com"));
@@ -349,7 +369,7 @@ mod tests {
 
     #[test]
     fn compile_globset_normalizes_apex_and_subdomains() {
-        let set = compile_globset(&["**.Example.COM.".to_string()]).unwrap();
+        let set = compile_denylist_globset(&["**.Example.COM.".to_string()]).unwrap();
 
         assert_eq!(true, set.is_match("example.com"));
         assert_eq!(true, set.is_match("api.example.com"));
@@ -357,7 +377,7 @@ mod tests {
 
     #[test]
     fn compile_globset_normalizes_bracketed_ipv6_literals() {
-        let set = compile_globset(&["[::1]".to_string()]).unwrap();
+        let set = compile_denylist_globset(&["[::1]".to_string()]).unwrap();
 
         assert_eq!(true, set.is_match("::1"));
     }