Enable FA linter rules for enforcing type annotations

[HyeockJinKim]

User Story

As a Backend.AI developer, I want to enable FA (flake8-annotations) rules to ensure complete type annotations across the codebase so that code quality and maintainability are improved.

Background

FA rules enforce type annotations on functions, methods, and variables. This ensures:

• All function parameters have type hints
• All function return types are annotated
• Class attributes and variables are properly typed
• No implicit Any types

Implementation Details

• Enable FA rules in ruff configuration (FA100, FA102, etc.)
• Add missing type annotations to functions and methods
• Add missing return type annotations
• Add type hints to class attributes and module-level variables
• Fix all existing FA violations across the codebase
• Update code style guidelines to require type annotations

Acceptance Criteria

• FA rules are enabled in ruff.toml
• All existing violations are fixed
• All functions have parameter and return type annotations
• Class attributes have type annotations
• CI/CD linter checks pass
• Code style documentation is updated

Technical Notes

• May be a large effort depending on existing violations
• Consider gradual rollout per package
• Use mypy to verify type correctness
• Avoid using Any type where possible

JIRA Issue: BA-4057