fix: fall back to vendored bubblewrap when system bwrap lacks --argv0 (#15338)

## Why

Fixes [#15283](https://github.com/openai/codex/issues/15283), where
sandboxed tool calls fail on older distro `bubblewrap` builds because
`/usr/bin/bwrap` does not understand `--argv0`. The upstream [bubblewrap
v0.9.0 release
notes](https://github.com/containers/bubblewrap/releases/tag/v0.9.0)
explicitly call out `Add --argv0`. Flipping `use_legacy_landlock`
globally works around that compatibility bug, but it also weakens the
default Linux sandbox and breaks proxy-routed and split-policy cases
called out in review.

The follow-up Linux CI failure was in the new launcher test rather than
the launcher logic: the fake `bwrap` helper stayed open for writing, so
Linux would not exec it. This update also closes the user-visibility gap
from review by surfacing the same startup warning when `/usr/bin/bwrap`
is present but too old for `--argv0`, not only when it is missing.

## What Changed

- keep `use_legacy_landlock` default-disabled
- teach `codex-rs/linux-sandbox/src/launcher.rs` to fall back to the
vendored bubblewrap build when `/usr/bin/bwrap` does not advertise
`--argv0` support
- add launcher tests for supported, unsupported, and missing system
`bwrap`
- write the fake `bwrap` test helper to a closed temp path so the
supported-path launcher test works on Linux too
- extend the startup warning path so Codex warns when `/usr/bin/bwrap`
is missing or too old to support `--argv0`
- mirror the warning/fallback wording across
`codex-rs/linux-sandbox/README.md` and `codex-rs/core/README.md`,
including that the fallback is the vendored bubblewrap compiled into the
binary
- cite the upstream `bubblewrap` release that introduced `--argv0`

## Verification

- `bazel test --config=remote --platforms=//:rbe
//codex-rs/linux-sandbox:linux-sandbox-unit-tests
--test_filter=launcher::tests::prefers_system_bwrap_when_help_lists_argv0
--test_output=errors`
- `cargo test -p codex-core system_bwrap_warning`
- `cargo check -p codex-exec -p codex-tui -p codex-tui-app-server -p
codex-app-server`
- `just argument-comment-lint`

codex-rs/tui_app_server/src/app.rs

@@ -435,8 +435,8 @@ fn emit_project_config_warnings(app_event_tx: &AppEventSender, config: &Config)
     )));
 }
 
-fn emit_missing_system_bwrap_warning(app_event_tx: &AppEventSender) {
-    let Some(message) = codex_core::config::missing_system_bwrap_warning() else {
+fn emit_system_bwrap_warning(app_event_tx: &AppEventSender) {
+    let Some(message) = codex_core::config::system_bwrap_warning() else {
         return;
     };
 
@@ -2794,7 +2794,7 @@ impl App {
         let (app_event_tx, mut app_event_rx) = unbounded_channel();
         let app_event_tx = AppEventSender::new(app_event_tx);
         emit_project_config_warnings(&app_event_tx, &config);
-        emit_missing_system_bwrap_warning(&app_event_tx);
+        emit_system_bwrap_warning(&app_event_tx);
         tui.set_notification_method(config.tui_notification_method);
 
         let harness_overrides =