UOW-015: Add Mocking for External Dependencies in Tests

[andhijeannot]

Overview

Phase: 5 - Testing
Priority: Medium
Estimated Effort: 6-8 hours

Problem Description

Current tests lack proper mocking of external dependencies (filesystem, network, OS calls), making them slower and less reliable.

Acceptance Criteria

• Filesystem operations mocked for unit tests
• Network operations mocked where applicable
• OS-specific calls abstracted and mockable
• Test utilities created for common mocking patterns
• Faster test execution through reduced I/O
• More reliable tests with predictable external dependencies

Files to Modify

• internal/testutil/mocks.go (new file)
• internal/testutil/filesystem.go (new file)
• All existing test files (update to use mocks)

See roadmap/phase5-testing/UOW-015-test-infrastructure.md for detailed implementation plan.