fix: UI sample_size ignored due to Planner LLM default override

[JoonYong-Park]

Summary

• Fixed a bug where the custom sample size set in the UI was always overridden by the Planner LLM's default value (100)
• The orchestrator now explicitly overrides sample_size in both plan_metadata.config and plan_yaml with the user's actual selection after receiving the planner response

Root Cause

In generate_leaderboard(), only query.query (the text string) was passed to planner_agent.create_evaluation_plan(). The user's query.sample_size (derived from the UI's sample_scale setting) was never forwarded. Since the Planner LLM prompt defaults to sample_size: 100, the UI setting was silently ignored.

Before / After

Before — Custom(10) set but 100 samples evaluated

After — Custom(7) set and 7 samples evaluated correctly