Add vendor/proprietary BLE parser extension examples, tests, and docs

[Copilot]

Closes the gap for consumers who need to register parsers for non-SIG (proprietary) UUIDs — previously undocumented and with no examples.

Changes

examples/vendor_parsers/register_parsers.py

Two real-world proprietary characteristic implementations:

• Nordic LBS (NordicLEDCharacteristic, NordicButtonCharacteristic) — single-byte LED/button state using real Nordic Semiconductor proprietary UUIDs from the nRF5 SDK LBS example
• Govee-style thermometer (GoveeThermometerCharacteristic) — 4-byte sint16 LE temp ÷ 100 °C + uint16 LE humidity ÷ 100 %, result typed as a msgspec.Struct

register_all() uses explicit register_custom_characteristic_class(..., override=True) rather than auto-registration, making it safe to call idempotently across any lifecycle phase (including test fixtures that clear registries between runs).

from examples.vendor_parsers.register_parsers import register_all, GOVEE_THERMO_UUID
import struct

translator = register_all()

payload = bytearray(struct.pack("<hH", 2250, 6510))
result = translator.parse_characteristic(GOVEE_THERMO_UUID, payload)
# GoveeThermometerReading(temperature=22.5, humidity=65.1)

tests/gatt/characteristics/test_vendor_parsers.py

20 tests covering parse, encode, round-trip, error cases (empty/short payload → CharacteristicParseError), and translator UUID dispatch for all three vendor characteristics. An autouse fixture calls register_all() before each test.

docs/source/how-to/usage.md

New "Vendor / Proprietary Parser Extensions" section with a 3-step walkthrough: define class → register → parse, with inline examples and links to the example script and tests.

Supporting changes

• pyproject.toml: added ARG002 to examples/** per-file-ignores — ctx/validate are required by the BaseCharacteristic interface even when unused in simple implementations
• test_characteristic_test_coverage.py: added test_vendor_parsers.py to _EXCLUDED_TEST_FILES (cross-cutting test, not characteristic-specific, same pattern as test_custom_characteristics.py)

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