feat(govee): add H5121 motion sensor contact support

[svc-finitelabs]

Summary

• add Govee H5121 motion sensor support to the Govee BLE parser
• use native Control4 AES-128-ECB for H512x payload decryption instead of any external runtime
• expose H5121 motion as a CONTACT_SENSOR in the esphome_govee driver

Details

This implementation is intentionally close to the Home Assistant / govee-ble parser behavior because the device is not available locally for validation.

Implemented behavior:

• detect and parse H5121 via the H512x encrypted advertisement path
• validate encrypted packet CRC
• decrypt the 16-byte payload using native C4:Decrypt("AES-128-ECB", ...) with the same reverse-key / reverse-data / reverse-output flow used by upstream govee-ble
• interpret H5121 motion from the decrypted event byte
• map motion to a dynamic CONTACT_SENSOR binding in Control4
• send OPENED on motion, then auto-send CLOSED after 30 seconds
• if another motion event arrives before timeout, restart the clear timer

Notes

• no Python or external decrypt helper is used
• this is still a best-effort testing PR intended to produce an artifact for real device validation
• ticket: DRV-40