Add edge case tests for prompt validation #316

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.