Add Solarbank 3 E2700 Pro support

[MrChurch]

Summary

Add direct local BLE support for the Solarbank 3 E2700 Pro (A17C5). This PR intentionally covers the device library only: it does not add Home Assistant entities, system-level Solarbank aggregation, or cloud metadata handling.

Protocol and controls

• Dynamic secp256r1 ECDH key pair per connection, followed by AES-GCM session authentication with the captured AAD.
• Authenticated 4022 account and 4027 client-security steps; the decrypted 4827 result is validated before the session is accepted.
• Encrypted 4040 telemetry requests, plus an authenticated 4030 firmware-information request.
• Local controls:
  • set_schedule(power_w): a uniform seven-day 405e schedule, 0-1200 W in 50 W steps.
  • set_schedule_mode(charge): writes the observed charge-slot direction; discharge remains the compatible default.
  • set_max_load(max_load_w): 4080 values 350, 600, 800, and 1200 W.

Follow-up stability update

The follow-up commit 0f27631 is based on repeated local A17C5 BLE captures and addresses stability after reconnects and firmware updates:

• Handle authenticated 4409 battery metadata and preserve it across normal reconnects.
• Accept both observed expansion-battery record markers: 6301 and the changed 6a01 marker seen on firmware 1.0.7.3.
• Correct the individual PV mapping to c6/c7/c8/c9; ca is not the fourth MPPT input.
• Reassemble only actual multi-fragment encrypted frames. In particular, a valid single encrypted frame beginning with 0x11 is no longer mistaken for a fragment and discarded.
• Merge partial authenticated telemetry updates instead of clearing existing values.
• Decode the 4830 firmware-information response and expose the verified Solarbank/component strings through firmware_versions.
• Include expansion battery SoC values in the aggregate battery percentage where available.

Validation

• python -m pytest tests/test_solarbank3.py -q: 8 passed.
• Targeted Ruff F/E checks pass for the changed Python files.
• Python bytecode compilation passes for SolixBLE and tests.
• git diff --check passes.
• Behaviour was verified against a standalone A17C5 Solarbank 3 E2700 Pro on firmware 1.0.7.1 and 1.0.7.3.

The implementation is deliberately scoped to the standalone Solarbank 3 device. System integration and Home Assistant-specific code are not part of this library PR.