fix(evaluation): open eval/session JSON files as UTF-8

[moneebarif1]

Summary

Four open() calls in the evaluation module read/write JSON eval data and session files without an explicit encoding, so they fall back to the platform default (e.g. cp1252 on Windows). Eval datasets and sessions routinely contain non-ASCII text (model prompts/responses in other languages, emoji), so on non-UTF-8 locales these calls raise UnicodeDecodeError on read or write mojibake on save.

Changes

Pass encoding="utf-8" to make behavior deterministic across platforms:

• evaluation/agent_evaluator.py
  • load_json() — reads eval/test JSON
  • _generate_responses... — writes regenerated eval data
  • _get_initial_session() — reads the initial session JSON
• evaluation/evaluation_generator.py
  • reads the session file before generating responses

This matches the rest of the evaluation module, which already opens files with encoding="utf-8" (local_eval_sets_manager.py, eval_config.py, local_eval_set_results_manager.py, and one existing call in agent_evaluator.py itself) — so these four were inconsistent oversights.

Testing

No behavioral change on UTF-8-default platforms; the fix removes locale-dependent failures on Windows/other locales. Existing evaluation tests continue to pass.

[ankursharmas]

This change is already checked in.