fix(sandbox): fix bwrap lookup for multi-entry PATH (#15973)

## Summary
- split the joined `PATH` before running system `bwrap` lookup
- keep the existing workspace-local `bwrap` skip behavior intact
- add regression tests that exercise real multi-entry search paths

## Why
The PATH-based lookup added in #15791 still wrapped the raw `PATH`
environment value as a single `PathBuf` before passing it through
`join_paths()`. On Unix, a normal multi-entry `PATH` contains `:`, so
that wrapper path is invalid as one path element and the lookup returns
`None`.

That made Codex behave as if no system `bwrap` was installed even when
`bwrap` was available on `PATH`, which is what users in #15340 were
still hitting on `0.117.0-alpha.25`.

## Impact
System `bwrap` discovery now works with normal multi-entry `PATH` values
instead of silently falling back to the vendored binary.

Fixes #15340.

## Validation
- `just fmt`
- `cargo test -p codex-sandboxing`
- `cargo test -p codex-linux-sandbox`
- `just fix -p codex-sandboxing`
- `just argument-comment-lint`

codex-rs/core/tests/suite/approvals.rs

@@ -179,7 +179,7 @@ impl ActionKind {
                 Ok((event, Some(command)))
             }
             ActionKind::RunCommand { command } => {
-                let event = shell_event(call_id, command, 1_000, sandbox_permissions)?;
+                let event = shell_event(call_id, command, 2_000, sandbox_permissions)?;
                 Ok((event, Some(command.to_string())))
             }
             ActionKind::RunUnifiedExecCommand {