Improve test coverage for src/core.ts security-critical functions

Objective

Improve test coverage for security-critical functions in src/core.ts to meet the 90% coverage target.

Current Coverage Status

• Lines: 27.8% (target: 90%)
• Functions: 33.3% (target: 90%)
• Branches: 28.6% (target: 90%)

Security-Critical Functions Requiring Coverage

• createAuthenticatedRequest - Currently covered but needs more edge cases
• handleApiResponse - Currently covered but needs error scenarios
• encodeProjectId - Currently not covered (0% function coverage)
• trackProjectIdUsage - Currently not covered
• getProjectIdUsageStats - Currently not covered
• buildGitLabUrl - Currently not covered

Tasks

• Write comprehensive tests for encodeProjectId function
• Add tests for trackProjectIdUsage analytics
• Add tests for getProjectIdUsageStats reporting
• Add tests for buildGitLabUrl URL construction
• Add error scenario tests for createAuthenticatedRequest
• Add error scenario tests for handleApiResponse
• Test edge cases for token validation logic
• Test various GitLab API URL configurations

Acceptance Criteria

• Line coverage ≥ 80% (progressive toward 90%)
• Function coverage ≥ 80% (progressive toward 90%)
• Branch coverage ≥ 80% (progressive toward 90%)
• All security-critical functions have comprehensive test cases
• Error scenarios and edge cases are covered

Priority

High - Security-critical code should have comprehensive test coverage

Related

• Part of MR #7 (closed) coverage testing integration
• Contributes to overall project coverage goals