Add semantic validation

What does this MR do and why?

Adds semantic validation for dynamic input rules, catching configuration errors at parse time:

• references to undefined inputs in rule conditions
• circular dependencies

⛔ Invalid - undefined input:

  spec:
    inputs:
      region:
        rules:
          - if: '$[[ inputs.environment ]] == "prod"'  #'environment' not defined
            options: ['us', 'eu']

⛔ Invalid - undefined input:

  spec:
    inputs:
      a:
        rules:
          - if: '$[[ inputs.b ]] == "x"'
            options: ['y']
      b:
        rules:
          - if: '$[[ inputs.a ]] == "y"'  # cicular deps
            options: ['x']

Valid ✅

  spec:
    inputs:
      environment:
        options: ['dev', 'prod']
      region:
        rules:
          - if: '$[[ inputs.environment ]] == "prod"'
            options: ['us', 'eu']

References

References

#563376

MR	Status	Description
Input lexeme	!206891 (merged)	Parse $[[ inputs.X ]] syntax in expressions
Rules converter	In review: !209135	Convert expression AST to JSON
Semantic validation	👈 You are here	Validate input references and circular dependencies, error handlin
GraphQL API	In review next, needs: rules converter	Expose rules with condition trees

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