Support NFC-Ethernet pairing mode for python_testing (SDK) tests

Summary

Add support for the nfc-ethernet DUT pairing mode to the Test Harness, so python_testing (SDK) tests can commission Matter-over-Ethernet devices via NFC.

Background

• Upstream connectedhomeip PR project-chip/connectedhomeip#43613 adds an nfc-ethernet pairing subcommand to chip-tool. As of this writing it is still open (not merged) and has requested changes from a reviewer (OlivierGre) who disputes the approach of inferring "no network setup needed" purely from NFC transport type, rather than from the Network Commissioning cluster's feature map (see linked draft alternative #72240). Because of this, chip-tool itself does not yet support nfc-ethernet, and YAML/chip-tool-based TH tests can't use it yet.

• The companion PR, project-chip/connectedhomeip#43657 ("Add nfc-ethernet in Python controller"), is already merged and is present in our connectedhomeip submodule. It adds "nfc-ethernet" as a valid --commissioning-method / --in-test-commissioning-method choice in matter_testing_infrastructure/matter/testing/runner.py, plus ChipDeviceCtrl.CommissionNfcEthernet() and dispatch in matter/testing/commissioning.py. This means our SDK python_testing tests can already accept nfc-ethernet as a commissioning method — the TH's own config/dispatch layer just doesn't expose or map it yet.

Scope (this issue)

Add TH support for nfc-ethernet for python_testing (SDK) tests only — YAML/chip-tool test support should wait until upstream #43613 merges and its design settles.

Proposed changes

• backend/app/constants/shared_constants.py (and its cli mirror cli/th_cli/shared_constants.py): add NFC_ETHERNET = "nfc-ethernet" to DutPairingModeEnum, and add it to NFC_PAIRING_MODES.
• backend/test_collections/matter/sdk_tests/support/python_testing/models/utils.py (generate_command_arguments): NFC-Ethernet needs neither --wifi-* nor --thread-dataset-hex args (unlike nfc-wifi/nfc-thread) — it should fall straight through to the existing NFC handling that suppresses --discriminator/--passcode and injects NFC_Reader_index.
• backend/test_collections/matter/test_environment_config.py: confirm NetworkConfig validation doesn't require wifi/thread config when pairing_mode is nfc-ethernet (currently NetworkConfig.wifi/.thread appear to be required fields regardless of pairing mode — may need to become optional or conditionally validated).
• backend/test_collections/matter/sdk_tests/support/python_testing/models/test_suite.py: no OTBR/border-router setup needed for nfc-ethernet (already excluded, just confirm).
• cli/th_cli/config.py PairingMode enum / VALID_PAIRING_MODES: add NFC_ETHERNET for parity (currently unused for validation but should stay in sync with the backend enum).
• Update tests mirroring the nfc-wifi/nfc-thread coverage in backend/test_collections/matter/sdk_tests/support/tests/python_tests/test_utils.py and backend/test_collections/matter/sdk_tests/support/tests/matter/test_test_environment_config.py.
• Update docs: backend/test_collections/matter/sdk_tests/support/tests/docs/TestEnvironmentConfigMatter-scenarios.md.

Out of scope (tracked separately / blocked upstream)

• matter_yaml_runner.py / chip_suite.py (YAML/chip-tool pairing) — blocked on chip-tool itself supporting nfc-ethernet via upstream PR #43613.