fix: trigger downstream experiment validation

[Pigbibi]

Summary

• run upstream experiment validation on the self-hosted runner where Binance history downloads already work
• toggle a downstream experiment-validation label so BinancePlatform can start validation from the issue event path
• keep the downstream workflow-dispatch attempt as a non-fatal fallback

Testing

• python3 -m unittest tests/test_prepare_experiment_validation.py tests/test_render_experiment_validation_summary.py tests/test_post_experiment_validation_comment.py tests/test_experiment_validation_workflow_config.py tests/test_monthly_optimization_planner_workflow_config.py
• uvx ruff check scripts tests
• ruby -e 'require "yaml"; [".github/workflows/experiment_validation.yml", ".github/workflows/monthly_optimization_planner.yml"].each { |p| YAML.load_file(p); puts "parsed #{p}" }'