fix(types): preserve logprob token bytes from server

[vineoy]

Fixes #750.

Server docs/openapi.yaml returns bytes[] per token in Logprob/TokenLogprob, but TokenLogprob only modeled token/logprob, so raw token bytes were silently dropped.

Additive Optional bytes field on TokenLogprob (inherited by Logprob), defaults None so old payloads still parse. Matches existing style (type/input/format already shadow builtins) and follows #601 logprobs pattern.

Verification:

• parsing with bytes, missing-bytes default, nested GenerateResponse, dict-style access all checked
• pytest tests/test_client.py -k logprob: 2 passed
• ruff check + format: clean
• full tests/test_utils + test_type_serialization: 28 passed, 2 pre-existing image-path failures also present on clean main