Fix flake8 violations blocking PyLint/build (3.11) workflow

[Copilot]

The PyLint / build (3.11) Actions job was failing due to flake8 errors introduced in backend service files. This PR removes the specific lint violations causing the workflow failure without changing runtime behavior.

• Lint blocker in chat_service.py

  • Removed an unused import (extract_usage_from_stream_chunk) from common.logging.llm_token_telemetry.

• Whitespace violations in history_service.py

  • Removed trailing-whitespace-only blank lines flagged by flake8 (W293) in the title-generation path.

from common.logging.llm_token_telemetry import (
    TokenUsageScope,
    extract_usage_from_dict,
)