perf(orchestrator): replace ip netns exec with nsenter to halve mount namespace copies on sandbox startup

[AdaAibaby]

#3012
p.configure() latency under 100+ concurrent sandboxes:

/cc @jakubno @dobrac @ValentaTomas Would appreciate your review on this change, thanks!

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[gemini-code-assist]

Code Review

This pull request replaces the use of external unshare and ip netns exec commands with syscall.CLONE_NEWNS and nsenter --net to optimize performance by avoiding redundant mount namespace copies. It also updates the start script templates and adds comprehensive unit tests to verify the new execution path and command formatting. I have no feedback to provide.

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[dobrac]

Thank you for the contribution!

Can you please verify the mount propagation is still set to private as with unshare -m? I believe the syscall.CLONE_NEWNS does not carry the same isolation.

[AdaAibaby]

@dobrac — verified on the orchestrator host (kernel 6.8.0-55-generic, util-linux 2.39.3)
using bpftrace to trace the mount tracepoint at the kernel syscall boundary:

sudo bpftrace -e '
tracepoint:syscalls:sys_enter_unshare /args->unshare_flags & 131072/ {
    printf("[%s pid=%d] unshare(CLONE_NEWNS=0x%x)\n", comm, pid, args->unshare_flags);
}
tracepoint:syscalls:sys_enter_mount /(args->flags & 278528) == 278528/ {
    printf("[%s pid=%d] mount(target=%s, MS_REC|MS_PRIVATE=0x%x)\n",
           comm, pid, str(args->dir_name), args->flags);
}
'

Running both paths concurrently:

=== path A: old — unshare -m ===
[unshare pid=3291031] unshare(CLONE_NEWNS=0x20000)
[unshare pid=3291031] mount(target=/, MS_REC|MS_PRIVATE=0x44000)

=== path B: new — CLONE_NEWNS via SysProcAttr + script-first mount --make-rprivate / ===
[unshare pid=3291033] unshare(CLONE_NEWNS=0x20000)
[mount   pid=3291033] mount(target=/, MS_REC|MS_PRIVATE=0x44000)

Both paths produce the identical kernel syscall sequence — unshare(CLONE_NEWNS=0x20000)
followed immediately by mount(MS_REC|MS_PRIVATE=0x44000, target=/). The only difference
is which binary issues the second call: in the old path the unshare binary does it
implicitly before exec; in the new path the mount --make-rprivate / at the top of the
start script does it explicitly. The resulting propagation state is identical.

Also updated 5b55eed to correct the Unshareflags comment (CLONE_NEWNS alone does not
carry MS_PRIVATE; private isolation depends on the explicit mount --make-rprivate / as
the invariant first command of both script templates).