Report enum variable validation errors cleanly

[httpdss]

Summary

• Add TemplateVariableError for missing/invalid template variable validation failures.
• Validate provided --vars values against declared variable schema, so enum mismatches are caught before rendering.
• Catch template variable validation failures in structkit generate and exit with a clean ❗ ... message instead of a traceback.

Verification

• pre-commit run --all-files
• pytest -q → 188 passed
• git diff --check
• Focused CLI reproduction for both invalid and missing enum values confirmed exit code 1 with no Traceback and clean messages:
  • Variable 'environment' must be one of: dev, staging, prod. Got: qa.
  • Variable 'environment' must be set to one of: dev, staging, prod. No value was provided. Pass --vars environment=VALUE or define a default.