fix: fix old system bubblewrap compatibility without falling back to vendored bwrap (#15693)

Fixes #15283.

## Summary
Older system bubblewrap builds reject `--argv0`, which makes our Linux
sandbox fail before the helper can re-exec. This PR keeps using system
`/usr/bin/bwrap` whenever it exists and only falls back to vendored
bwrap when the system binary is missing. That matters on stricter
AppArmor hosts, where the distro bwrap package also provides the policy
setup needed for user namespaces.

For old system bwrap, we avoid `--argv0` instead of switching binaries:
- pass the sandbox helper a full-path `argv0`,
- keep the existing `current_exe() + --argv0` path when the selected
launcher supports it,
- otherwise omit `--argv0` and re-exec through the helper's own
`argv[0]` path, whose basename still dispatches as
`codex-linux-sandbox`.

Also updates the launcher/warning tests and docs so they match the new
behavior: present-but-old system bwrap uses the compatibility path, and
only absent system bwrap falls back to vendored.

### Validation

1. Install Ubuntu 20.04 in a VM
2. Compile codex and run without bubblewrap installed - see a warning
about falling back to the vendored bwrap
3. Install bwrap and verify version is 0.4.0 without `argv0` support
4. run codex and use apply_patch tool without errors

<img width="802" height="631" alt="Screenshot 2026-03-25 at 11 48 36 PM"
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<img width="807" height="634" alt="Screenshot 2026-03-25 at 11 47 32 PM"
src="https://github.com/user-attachments/assets/5af8b850-a466-489b-95a6-455b76b5050f"
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<img width="812" height="635" alt="Screenshot 2026-03-25 at 11 45 45 PM"
src="https://github.com/user-attachments/assets/438074f0-8435-4274-a667-332efdd5cb57"
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<img width="801" height="623" alt="Screenshot 2026-03-25 at 11 43 56 PM"
src="https://github.com/user-attachments/assets/0dc8d3f5-e8cf-4218-b4b4-a4f7d9bf02e3"
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---------

Co-authored-by: Michael Bolin <mbolin@openai.com>

codex-rs/linux-sandbox/README.md

@@ -8,23 +8,24 @@ This crate is responsible for producing:
   - this should also be true of the `codex` multitool CLI
 
 On Linux, the bubblewrap pipeline prefers the system `/usr/bin/bwrap` whenever
-it is available and supports the required argv-rewrite flags. If `/usr/bin/bwrap`
-is missing or too old to support the required flags, the helper falls back to
-the vendored bubblewrap path compiled into this binary.
-Codex also surfaces a startup warning when `/usr/bin/bwrap` is missing or too
-old to support the required flags so users know it is falling back to the
-vendored helper.
+it is available. If `/usr/bin/bwrap` is present but too old to support
+`--argv0`, the helper keeps using system bubblewrap and switches to a
+no-`--argv0` compatibility path for the inner re-exec. If `/usr/bin/bwrap` is
+missing, the helper falls back to the vendored bubblewrap path compiled into
+this binary.
+Codex also surfaces a startup warning when `/usr/bin/bwrap` is missing so users
+know it is falling back to the vendored helper.
 
 **Current Behavior**
 - Legacy `SandboxPolicy` / `sandbox_mode` configs remain supported.
 - Bubblewrap is the default filesystem sandbox pipeline.
-- If `/usr/bin/bwrap` is present and supports the required argv-rewrite flags,
-  the helper uses it.
-- If `/usr/bin/bwrap` is missing or too old to support the required flags, the
-  helper falls back to the vendored bubblewrap path.
-- If `/usr/bin/bwrap` is missing or too old to support the required flags,
-  Codex also surfaces a startup warning instead of printing directly from the
-  sandbox helper.
+- If `/usr/bin/bwrap` is present, the helper uses it.
+- If `/usr/bin/bwrap` is present but too old to support `--argv0`, the helper
+  uses a no-`--argv0` compatibility path for the inner re-exec.
+- If `/usr/bin/bwrap` is missing, the helper falls back to the vendored
+  bubblewrap path.
+- If `/usr/bin/bwrap` is missing, Codex also surfaces a startup warning instead
+  of printing directly from the sandbox helper.
 - 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`)