fix: discover the MPS pipe directory for IPC mounts - #2143

Description

NewIPCDiscoverer intends to mount the MPS pipe directory (/tmp/nvidia-mps) alongside the persistenced and fabricmanager sockets, but it located it with a file locator. The file locator's default filter (assertFile) rejects directories, and the miss is only logged at debug level, so the MPS pipe directory was never found and no mount was emitted for it in generated CDI specs or the runtime's automatic CDI modifications.

This change uses the existing directory locator for the MPS pipe directory. The socket lookup and the IPC mount options (nosuid, nodev, rbind, rprivate, noexec) are unchanged.

The legacy (libnvidia-container) path is not affected; it already locates the MPS pipe directory in nvc_info.c.

Known gaps, not addressed here:

• libnvidia-container honours CUDA_MPS_PIPE_DIRECTORY; the Go discoverer still only checks /tmp/nvidia-mps.
• The directory is resolved relative to the driver root, matching libnvidia-container. With a containerized driver root, the MPS daemon's pipe directory is likely on the host's /tmp and may still not be found.

Checklist

• No secrets, sensitive information, or unrelated changes
• Unit tests passing (make test)
• Lint checks passing (make lint)
• Test cases are added for new code paths
• Commits are signed-off and cryptographically signed

Testing

• Added TestNewIPCDiscoverer, a table test over a temporary driver root covering: no IPC paths present, the MPS directory (emitted with the IPC mount options), a regular file at the MPS path (ignored), sockets under /run and /var/run, socket paths that are directories (ignored), and sockets together with MPS. The MPS case fails with the previous file locator.
• go test ./internal/discover/... ./pkg/lookup/..., go vet, and golangci-lint run on the touched packages pass.
• Not yet tested on a GPU node running the MPS control daemon.

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