fix: nightly hardening - CLI config top-level validation

[mouse-value-add]

problem

promptlens run assumes the YAML config always parses into a mapping. Empty files (None) or non-mapping top-level YAML (for example a list) fail later with less clear behavior and weaker guardrails.

approach

• Load config with explicit UTF-8 encoding.
• Add early top-level validation in CLI run flow:
  • Reject empty YAML config files.
  • Reject non-mapping top-level config shapes.
• Return clear, actionable validation messages before attempting RunConfig parsing.
• Add regression tests for both edge cases.

verification

• Ran test suite: python3 -m pytest -q
• Result: 11 passed.
• Added tests:
  • test_run_rejects_empty_config_file
  • test_run_rejects_non_mapping_top_level_config

risks

• Low risk. Behavior only changes for invalid config inputs.
• Valid config paths are unchanged.

rollback plan

• Revert commit e2ab2a8 to restore previous behavior.