feat: add input-side incident validation before PDF fill

[geoffkats]

Description

Adds input-side validation for required incident fields (incident_type, location, time) before PDF generation to prevent incomplete data from entering the fill pipeline.

Fixes #305

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Unit tests: 27 test cases covering valid input, missing fields, empty values, whitespace-only values, and invalid input types
• Integration: Validator integrated into FileManipulator pipeline
• Python 3.14.2 environment verified

Test Configuration:

• Python: 3.14.2
• Test Suite: pytest
• Command: .venv\Scripts\python.exe -m pytest tests/test_validator.py -v
• Result: 27/27 PASSED

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Related

• Distinct from issue [FEAT]: Schema Validation + Error Recovery #114 — this validates input completeness (required fields present and non-empty) before LLM runs. Issue [FEAT]: Schema Validation + Error Recovery #114 addresses output schema validation (type correctness, confidence scores) after LLM extraction