Add edge case tests for prompt validation

## Problem
The prompt validation test suite lacks coverage for important edge cases that could cause unexpected behavior in production.

## Proposed Solution
Add three new test cases to `tests/prompt_validation_test.py` to improve test coverage:

1. **Empty extraction text** - Verify that empty strings are properly rejected
2. **Unicode and special characters** - Ensure proper handling of café, °F, and other unicode
3. **Very long extraction text** - Test graceful handling of 1000+ character extractions

## Benefits
- Improves reliability by catching edge case bugs early
- Better test coverage for error handling paths
- Documents expected behavior for edge cases

## Implementation
Three new test methods added to `PromptAlignmentValidationTest` class following existing test patterns.

Open tests/prompt_validation_test.py and read the existing PromptAlignmentValidationTest methods and their assertions. Add coverage for empty extraction text, Unicode such as café and °F, and extraction text over 1000 characters, following the established patterns. Run the prompt validation tests; done means all three edge cases have explicit passing assertions.

Written by the indexing model from the issue text.