Add telnet control (set_telnet/telnet_enabled + CLI)

[widgetii]

Summary

Adds control of the built-in Dahua telnet server via the DHIP binary protocol (TCP 5000), mirroring the flow in the open_telnet.sh reference script but using the existing configManager.setConfig machinery instead of HTTP RPC2.

Verified on hardware

Device: 10.216.128.118 (SD-2N-4G-class camera), admin / admin54321.

Action	Result
set_telnet(True)	port 23 opened (confirmed with nc -zv)
set_telnet(False)	port 23 closed (connection refused)
round-trip read	telnet_enabled() matches written state

Before/after InnerServer config read back from the device:

before: {'Telnet': {'Enable': False}}
after:  {'Telnet': {'Enable': True}}

Public API

cam.set_telnet(True)        # enable
cam.set_telnet(False)       # disable
cam.telnet_enabled()        # -> bool
cam.get_inner_server_config()  # raw InnerServer table

set_telnet does a read-modify-write of InnerServer.Telnet so sibling subkeys (SSH, FTP, ...) are preserved.

CLI

dhip HOST telnet        # prints on / off
dhip HOST telnet on     # enable
dhip HOST telnet off    # disable

Sync / async

Mirrored in both DahuaClient (sync) and AsyncDahuaClient (async), per the CLAUDE.md convention.

Tests

• tests/test_dahua.TestTelnet.test_set_telnet_read_modify_write — fake-server round-trip; verifies the SSH subkey is preserved across enable/disable.
• python -m unittest discover -s tests — 30 passed
• python tests/test_loopback.py — OK

Checklist

• python -m unittest discover -s tests passes; tests/test_loopback.py passes.
• New public API has a test and a CLI flag.
• Sync and async surfaces stay consistent.
• No new third-party dependencies; no secrets in code.
• Verified on hardware (model/firmware noted above).