fix(build): raise file limit for native gateway builds

[purp]

Summary

Raise the macOS per-process file-descriptor limit before native local gateway builds so sccache does not fail with Too many open files.

Related Issue

No issue required: obvious localized macOS build reliability fix; generalizes the existing #2307 file-descriptor-limit safeguard to native gateway builds.

Changes

• Apply the shared build-environment limit guard to native Cargo gateway builds.
• Cover direct gateway and Docker, Podman, and VM gateway entrypoints.
• Add a native-Cargo regression case and update build architecture documentation.

Testing

• mise run test:build-env
• mise run pre-commit
• mise run test
• E2E tests (not applicable; build-environment-only change)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

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