feat: implement fail-fast dry-run validation for calculation templates

[eli-lindner-sj]

This PR introduces a new TemplateValidator service that performs a "dry-run" validation of the calculation template immediately upon starting a batch job. By verifying structural integrity
before spinning up worker processes, it prevents long-running batches from failing mid-execution due to configuration or template errors.

Key Changes

• Fail-Fast Template Validation (TemplateValidator):
  ** Intercepts the batch process early to verify that all sheets referenced by InputFields and OutputFields actually exist in the target Excel template.
  ** Provides aggregated, human-readable error messages detailing exactly which sheets or ranges are missing, allowing users to correct their configuration or template immediately.
• The validator opens the template with FileShare.ReadWrite (non-locking shared access), allowing validation to succeed even if the user currently has the template open in Excel.
• Extracted and centralized the logic for parsing OpenXML Defined Names (BuildDefinedNamesLookup). This ensures the new validator and the existing execution engine
  (CalculationHeaderWriter) use identical rules to identify valid, writable cell references (e.g., Sheet!$A$1), ignoring complex formulas/offsets.
• The validator aligns with the existing engine's leniency: while missing data sheets are fatal, missing HeaderInputs are silently ignored, ensuring we do not break workflows relying on generic configs across varying templates.