Add Linux NPU discovery through sysfs accel devices

[the0cp]

Description

This PR adds Linux NPU discovery through sysfs accel devices

Currently, DeviceDiscovery::DiscoverDevicesForPlatform() on Linux discovers CPU and GPU devices, but NPU discovery is still missing. As a result, plugin execution providers that filter devices by OrtHardwareDeviceType_NPU do not receive any NPU hardware devices on Linux, even when the NPU is present and exposed by the kernel.

This change scans /sys/class/accel for accelN devices and creates OrtHardwareDevice entries with:

• type = OrtHardwareDeviceType_NPU
• PCI vendor_id
• PCI device_id
• accel_idx metadata
• pci_bus_id metadata when available

This enables Linux systems with NPUs exposed through the accel subsystem, such as AMD Ryzen AI / XDNA devices, to be reported through ORT device discovery and made available to plugin EP factories.

Changes

• Add Linux sysfs discovery for NPU devices under /sys/class/accel.
• Read NPU PCI vendor and device IDs from the underlying sysfs device path.
• Add NPU metadata including accel_idx and pci_bus_id.
• Include discovered NPU devices in DeviceDiscovery::DiscoverDevicesForPlatform().
• Add a kSysfsAccelPath constant for the Linux accel sysfs path.

Motivation

Linux plugin EPs that target NPUs rely on ORT passing OrtHardwareDeviceType_NPU devices into GetSupportedDevices(). Without Linux NPU discovery, those EPs cannot claim NPU devices and provider selection policies such as PREFER_NPU silently fall back to CPU.

Fixes #28660.