Add type hints to public API functions in the reports module

[himanshu231204]

Description

Several public functions in the \openagent_eval/reports/\ module are missing type hints. Adding proper type hints improves IDE autocompletion and catches type-related bugs early.

What to Do

Add or improve type hints for:

• \openagent_eval/reports/terminal.py\
• \openagent_eval/reports/markdown.py\
• \openagent_eval/reports/html.py\
• \openagent_eval/reports/json_report.py\
• \openagent_eval/reports/comparison.py\
• \openagent_eval/reports/manager.py\
• \openagent_eval/reports/base.py\

Focus on function signatures (parameters and return types). Use existing typed functions in the codebase as reference.

Good First Issue ✅

A straightforward code quality task. Great for learning Python type hints.