fix: resolve 500 error on /schema and add extra validation tasks

fix: standardize grader structure to tasks/ directory for OpenEnv validation compliance

fix: add mandatory /metadata and /schema endpoints for OpenEnv validation and update task metadata

Fix OpenEnv validation: Use explicit python grader for tasks

fix: task routing by name, remove out-of-range rewards, add grader field to tasks

fix: strictly bound task scores to satisfy OpenEnv Phase 2 testing bounds

fix: allow empty body for /reset endpoint to pass auto-grader

feat: CodeSensei - GRPO-trained LLM code debugger on OpenEnv