test: Strengthen pure Pluto DKG runner validation

[therustmonk]

This PR improves the existing pure Pluto DKG runner coverage.

The runner now explicitly verifies the expected validator count in the generated artifacts, making the semantic validation stricter and less implicit. It also allows pure-Pluto runs to execute without requiring Charon to be installed when no Charon nodes are configured.

Additionally, the DKG runner documentation and CI job naming were clarified so the pure Pluto 4-node threshold-3 flow is easier to identify and run locally.

Checklist

• Added explicit expected validator count validation to the DKG semantic verifier.
• Avoided requiring Charon for pure-Pluto runner executions.
• Clarified the CI job name for the pure Pluto DKG runner case.
• Documented how to run the pure Pluto DKG flow locally.
• Verified the runner locally with 4 Pluto nodes, threshold 3, and one validator.