fix(linux-sandbox): prefer system /usr/bin/bwrap when available (#14963)

## Problem
Ubuntu/AppArmor hosts started failing in the default Linux sandbox path
after the switch to vendored/default bubblewrap in `0.115.0`.

The clearest report is in
[#14919](https://github.com/openai/codex/issues/14919), especially [this
investigation
comment](https://github.com/openai/codex/issues/14919#issuecomment-4076504751):
on affected Ubuntu systems, `/usr/bin/bwrap` works, but a copied or
vendored `bwrap` binary fails with errors like `bwrap: setting up uid
map: Permission denied` or `bwrap: loopback: Failed RTM_NEWADDR:
Operation not permitted`.

The root cause is Ubuntu's `/etc/apparmor.d/bwrap-userns-restrict`
profile, which grants `userns` access specifically to `/usr/bin/bwrap`.
Once Codex started using a vendored/internal bubblewrap path, that path
was no longer covered by the distro AppArmor exception, so sandbox
namespace setup could fail even when user namespaces were otherwise
enabled and `uidmap` was installed.

## What this PR changes
- prefer system `/usr/bin/bwrap` whenever it is available
- keep vendored bubblewrap as the fallback when `/usr/bin/bwrap` is
missing
- when `/usr/bin/bwrap` is missing, surface a Codex startup warning
through the app-server/TUI warning path instead of printing directly
from the sandbox helper with `eprintln!`
- use the same launcher decision for both the main sandbox execution
path and the `/proc` preflight path
- document the updated Linux bubblewrap behavior in the Linux sandbox
and core READMEs

## Why this fix
This still fixes the Ubuntu/AppArmor regression from
[#14919](https://github.com/openai/codex/issues/14919), but it keeps the
runtime rule simple and platform-agnostic: if the standard system
bubblewrap is installed, use it; otherwise fall back to the vendored
helper.

The warning now follows that same simple rule. If Codex cannot find
`/usr/bin/bwrap`, it tells the user that it is falling back to the
vendored helper, and it does so through the existing startup warning
plumbing that reaches the TUI and app-server instead of low-level
sandbox stderr.

## Testing
- `cargo test -p codex-linux-sandbox`
- `cargo test -p codex-app-server --lib`
- `cargo test -p codex-tui-app-server
tests::embedded_app_server_start_failure_is_returned`
- `cargo clippy -p codex-linux-sandbox --all-targets`
- `cargo clippy -p codex-app-server --all-targets`
- `cargo clippy -p codex-tui-app-server --all-targets`

codex-rs/linux-sandbox/README.md

@@ -7,13 +7,20 @@ This crate is responsible for producing:
   - the `codex-exec` CLI can check if its arg0 is `codex-linux-sandbox` and, if so, execute as if it were `codex-linux-sandbox`
   - this should also be true of the `codex` multitool CLI
 
-On Linux, the bubblewrap pipeline uses the vendored bubblewrap path compiled
-into this binary.
+On Linux, the bubblewrap pipeline prefers the system `/usr/bin/bwrap` whenever
+it is available. If `/usr/bin/bwrap` is missing, the helper still 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 and is standardized on
-  the vendored path.
+- Bubblewrap is the default filesystem sandbox pipeline.
+- If `/usr/bin/bwrap` is present, the helper uses it.
+- 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`)