fix(runc-shim): support realtime signals for exec processes

[immanuwell]

Exec kills panic for Linux realtime signals because nix::Signal rejects them.
Use libc::kill so the kernel validates raw signal values.
Also, this matches the Go runc shim

Repro:

cargo build -p containerd-runc-shim --bin containerd-shim-runc-v2-rs
sudo ctr images pull docker.io/library/alpine:3.22
sudo ctr run --runtime "$PWD/target/debug/containerd-shim-runc-v2-rs" --null-io --detach docker.io/library/alpine:3.22 rt-signal sleep 300
sudo ctr tasks exec --exec-id sleeper --detach rt-signal sleep 300
sudo ctr tasks kill --exec-id sleeper --signal 34 rt-signal

34 is SIGRTMIN on x86_64 Linux.
Added a regression test too

[mxpv]

Needs import cleanup:

error: unused import: `Command as StdCommand`
   --> crates/runc-shim/src/runc.rs:851:19
    |
851 |         process::{Command as StdCommand, ExitStatus},
    |                   ^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D unused-imports` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(unused_imports)]`

error: unused import: `super::signal_process`
   --> crates/runc-shim/src/runc.rs:858:9
    |
858 |     use super::signal_process;
    |         ^^^^^^^^^^^^^^^^^^^^^

[Copilot]

Pull request overview

Updates exec-process signaling to support Linux realtime signals.

Changes:

• Replaces nix::Signal conversion with raw libc::kill.
• Adds a realtime-signal regression test.

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