feat: introducing a network sandbox proxy (#8442)

This add a new crate, `codex-network-proxy`, a local network proxy
service used by Codex to enforce fine-grained network policy (domain
allow/deny) and to surface blocked network events for interactive
approvals.

- New crate: `codex-rs/network-proxy/` (`codex-network-proxy` binary +
library)
- Core capabilities:
  - HTTP proxy support (including CONNECT tunneling)
  - SOCKS5 proxy support (in the later PR)
- policy evaluation (allowed/denied domain lists; denylist wins;
wildcard support)
  - small admin API for polling/reload/mode changes
- optional MITM support for HTTPS CONNECT to enforce “limited mode”
method restrictions (later PR)

Will follow up integration with codex in subsequent PRs.

## Testing

- `cd codex-rs && cargo build -p codex-network-proxy`
- `cd codex-rs && cargo run -p codex-network-proxy -- proxy`

.github/workflows/rust-ci.yml

@@ -265,11 +265,11 @@ jobs:
         name: Install musl build tools
         env:
           DEBIAN_FRONTEND: noninteractive
+          TARGET: ${{ matrix.target }}
+          APT_UPDATE_ARGS: -o Acquire::Retries=3
+          APT_INSTALL_ARGS: --no-install-recommends
         shell: bash
-        run: |
-          set -euo pipefail
-          sudo apt-get -y update -o Acquire::Retries=3
-          sudo apt-get -y install --no-install-recommends musl-tools pkg-config
+        run: bash "${GITHUB_WORKSPACE}/.github/scripts/install-musl-build-tools.sh"
 
       - name: Install cargo-chef
         if: ${{ matrix.profile == 'release' }}