fix: warn when bwrap cannot create user namespaces (#15893)

## Summary
- add a Linux startup warning when system `bwrap` is present but cannot
create user namespaces
- keep the Linux-specific probe, sandbox-policy gate, and stderr
matching in `codex-sandboxing`
- polish the missing-`bwrap` warning to point users at the sandbox
prerequisites and OS package-manager install path

## Details
- probes system `bwrap` with `--unshare-user`, `--unshare-net`, and a
minimal bind before command execution
- detects known bubblewrap setup failures for `RTM_NEWADDR`,
`RTM_NEWLINK`, uid-map permission denial, and `No permissions to create
a new namespace`
- preserves the existing suppression for sandbox-bypassed policies such
as `danger-full-access` and `external-sandbox`
- updates the Linux sandbox docs to call out the user-namespace
requirement

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/linux-sandbox/README.md

@@ -15,7 +15,8 @@ no-`--argv0` compatibility path for the inner re-exec. If `bwrap` is missing,
 the helper falls back to the vendored bubblewrap path compiled into this
 binary.
 Codex also surfaces a startup warning when `bwrap` is missing so users know it
-is falling back to the vendored helper.
+is falling back to the vendored helper. Codex surfaces the same startup warning
+path when bubblewrap cannot create user namespaces.
 
 **Current Behavior**
 - Legacy `SandboxPolicy` / `sandbox_mode` configs remain supported.
@@ -28,6 +29,8 @@ is falling back to the vendored helper.
   path.
 - If `bwrap` is missing, Codex also surfaces a startup warning instead of
   printing directly from the sandbox helper.
+- If bubblewrap cannot create user namespaces, Codex surfaces a startup warning
+  instead of waiting for a runtime sandbox failure.
 - Legacy Landlock + mount protections remain available as an explicit legacy
   fallback path.
 - Set `features.use_legacy_landlock = true` (or CLI `-c use_legacy_landlock=true`)