fix: add template validation for sablon 0.4.3 compatibility

Sablon 0.4.3 no longer raises Zip::Error for nonexistent template files,
instead returning an empty ZIP. This breaks the error handling in
CaseCourtReportsController that relies on catching Zip::Error to show
users a friendly "Template is not found" message.

Changes:
- Add file existence validation in CaseCourtReport#initialize
- Raise Zip::Error with descriptive message if template doesn't exist
- Update test to expect error during initialization (fail-fast approach)
- Maintain backward compatibility with existing error handling

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: compwron

app/models/case_court_report.rb

@@ -9,6 +9,7 @@ def initialize(path_to_template:, context:)
     @context = context
     # Validate template exists before processing (sablon 0.4+ no longer raises Zip::Error)
     raise Zip::Error, "Template file not found: #{path_to_template}" unless File.exist?(path_to_template)
+
     # NOTE: this is what is used for docx templates
     @template = Sablon.template(path_to_template)
   end