feat: Integrate py-opendisplay requirements & introduce library-backed BLEDeviceMetadata

[stuart-c-ai]

This PR adds the py-opendisplay==7.3.0 library as a dependency and introduces a new implementation of the BLEDeviceMetadata class that utilizes the library's models, while keeping the original implementation untouched.

Context & Motivation

• Goal: Transition toward only using the external py-opendisplay Python library for BLE communications, allowing the custom component's local BLE implementation to be deprecated and removed in the future.
• Minimal Impact: To minimize major changes during this transition, we are leaving the existing custom_components/opendisplay/ble/metadata.py file completely untouched.
• New Metadata Class: We have introduced a new BLEDeviceMetadata implementation under custom_components/opendisplay/metadata.py (outside the ble/ directory). This class dynamically deserializes the stored OpenDisplay configuration dictionary back into py-opendisplay config dataclasses (GlobalConfig, DisplayConfig, PowerOption, etc.), acting as a seamless wrapper.
• Compatibility: The new BLEDeviceMetadata is fully backward-compatible with ATC (flat) devices and utilizes opendisplay.ColorScheme as its color representation.
• Reference Migration: All internal imports across the custom component have been redirected to point to the new metadata class to ensure a seamless drop-in transition.

Verification & Testing

• Added comprehensive unit tests in tests/test_ble_metadata.py comparing the new library-backed implementation side-by-side with the old implementation for both ATC (flat) and OpenDisplay (nested config) devices.
• All unit tests pass successfully.