test: Add edge case tests for PDF TextLine class

[alxkm]

Description

This PR adds comprehensive edge case test coverage for TextLine class to ensure proper handling of boundary conditions and special character scenarios in PDF text layout processing.

Changes

Added test cases to TextLineTest:

• Zero line length: Verifies graceful handling of zero-length lines and character writing attempts
• Non-divisible line lengths: Tests integer division truncation behavior (e.g., 103/4)
• Boundary conditions: Tests minimum valid line lengths (1, 3, 4) and integer division edge cases
• Negative index handling: Ensures no crashes when character position is negative
• Non-printable characters: Tests handling of control characters (tab, newline, null)

Motivation

These tests address critical edge cases that could cause issues in production:

• Prevent crashes: Negative indices and zero-length lines are boundary conditions that must be handled gracefully
• Integer division edge cases: Line length calculations use integer division which can result in truncation
• Special character handling: PDFs may contain non-printable characters that need proper handling
• Robustness: These scenarios, while edge cases, can occur in real-world PDF processing

Testing

• All new tests pass successfully
• Tests focus on boundary conditions and error scenarios
• No production code changes required
• Tests follow existing patterns and conventions

[ilayaperumalg]

@alxkm Thanks for the PR!

[ilayaperumalg]

Rebased and merged as e90f1a0