Document CommandDialog validation gating

[woksin]

Summary

• document that custom CommandDialog inputs must keep command values synchronized for client validation
• add regression specs proving isValid is an additional gate and does not override command-form validity
• expand StepperCommandDialog validity specs for valid and invalid command-form states

Verification

• yarn test
• ../node_modules/.bin/tsc -p tsconfig.json -noEmit
• ../node_modules/.bin/eslint --cache --quiet .
• bash Documentation/verify-markdown.sh