Multiple fixes for BLE subscriptions

[DennisMoschina]

Reduced data races, improved timeout for reading, reduced BLE overhead

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[Copilot]

Pull request overview

This PR hardens BLE notification subscriptions and sensor-stream setup across the library to reduce subscription races, connection duplication, and improve OpenEarable V2 sensor-scheme reading reliability (including a notification→read fallback path).

Changes:

• Make BleGattManager.subscribe and SensorHandler.subscribeToSensorData async (Future<Stream<...>>) so callers can await notification readiness before listening.
• Rework several device streams to set up subscriptions in onListen, add closed-controller guards, and improve cancellation behavior.
• Improve OpenEarable V2 sensor-scheme loading by keeping a single notification subscription during scheme reads and adding synchronous-read/legacy fallbacks.

Reviewed changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
lib/src/utils/sensor_scheme_parser/v2_sensor_scheme_reader.dart	Adds buffered notification matching + read fallback for V2 sensor schemes.
lib/src/models/devices/polar_factory.dart	Awaits BLE subscription setup and guards stream controller usage.
lib/src/models/devices/open_ring.dart	Awaits BLE subscription setup and adds error-handling guards for battery/time sync.
lib/src/models/devices/open_earable_v2.dart	Moves subscribe/read setup into async onListen with closed-controller checks.
lib/src/models/devices/open_earable_v1.dart	Adds closed-controller guards + error propagation for battery/sensor streams.
lib/src/models/devices/open_earable_factory.dart	Awaits sensor subscription setup and guards controller writes.
lib/src/models/devices/esense_factory.dart	Awaits sensor subscription setup and adds cancellation + error guards.
lib/src/models/devices/cosinuss_one.dart	Awaits BLE subscription setup and adds controller lifecycle guards.
lib/src/models/devices/battery_gatt_reader/battery_level_status_service_gatt_reader.dart	Refactors polling via helper + unawaited, adds closed-controller guard.
lib/src/models/devices/battery_gatt_reader/battery_level_status_gatt_reader.dart	Refactors polling via helper + unawaited, adds closed-controller guard.
lib/src/models/devices/battery_gatt_reader/battery_health_status_gatt_reader.dart	Refactors polling via helper + unawaited, adds closed-controller guard.
lib/src/models/devices/battery_gatt_reader/battery_energy_status_gatt_reader.dart	Refactors polling via helper + unawaited, adds closed-controller guard.
lib/src/models/capabilities/sensor_specializations/open_ring/open_ring_sensor.dart	Makes OpenRing sensor subscription setup async and idempotent.
lib/src/managers/v2_sensor_handler.dart	Makes sensor subscription async and awaits BLE notification readiness.
lib/src/managers/sensor_handler.dart	Updates interface to Future<Stream<...>> for subscription readiness.
lib/src/managers/open_ring_sensor_handler.dart	Makes shared OpenRing data stream creation async and awaits BLE subscription.
lib/src/managers/open_earable_sensor_manager.dart	Makes sensor subscription async and awaits BLE notification readiness.
lib/src/managers/esense_sensor_handler.dart	Makes sensor subscription async and awaits BLE notification readiness.
lib/src/managers/ble_manager.dart	Adds pending connection reuse + async subscribe with setup/teardown tracking.
lib/src/managers/ble_gatt_manager.dart	Updates subscribe API contract to Future<Stream<...>>.
lib/open_earable_flutter.dart	Deduplicates wearable connection attempts by caching in-flight futures.
CHANGELOG.md	Documents breaking API changes and summarizes the BLE subscription fixes.

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