Add comprehensive tests for seasonal models and coverage analysis

[SZabolotnii]

This commit significantly improves test coverage for the seasonal models introduced in version 0.1.2, addressing a critical gap in the CRAN submission readiness.

New test file: tests/testthat/test-seasonal-models.R (500+ lines) ================================================================ Comprehensive testing for all seasonal model functionality:

SAR (Seasonal AutoRegressive) Models:

• Basic SAR(0,P)_s model fitting and validation
• Multiplicative SAR(p,P)_s models
• Coefficient bounds and convergence testing
• Multiple seasonal periods (quarterly, monthly)
• Mean/intercept parameter handling
• SARPMM2 S4 class structure validation

SMA (Seasonal Moving Average) Models:

• SMA(Q)_s model fitting with CSS and PMM2 methods
• Higher-order models (Q > 1)
• Method comparison and convergence parameters
• Innovation slot validation
• SMAPMM2 S4 class structure testing

SARMA (Combined Seasonal ARMA) Models:

• Full SARMA(p,P,q,Q)_s specification testing
• Pure seasonal and mixed models
• Order slot validation and residual properties
• SARMAPMM2 S4 class validation

SARIMA (Seasonal ARIMA) Models:

• Differencing order (d, D) handling
• Non-stationary data processing
• Full model specification testing
• SARIMAPMM2 S4 class structure

S4 Methods Testing:

• coef(), residuals(), fitted(), summary() methods
• Class inheritance from TS2fit
• Original series storage and moment statistics

Edge Cases & Integration:

• Short time series handling
• Large seasonal periods
• Constant series edge cases
• Comparison with stats::arima()

Total: 50+ distinct test cases covering all seasonal functionality

New coverage analysis script: run_coverage_analysis.R ====================================================== Automated script for comprehensive test coverage analysis:

• Package-wide coverage reporting with covr
• File-by-file coverage breakdown
• HTML report generation (coverage_report.html)
• Identification of low-coverage areas (<80%)
• Specific focus on seasonal model files
• Actionable recommendations based on coverage level
• Data export for CI/CD integration (coverage_data.rds)

New testing documentation: tests/TESTING_GUIDE.md
================================================== Complete guide for developers and contributors:

• Test structure and organization overview
• Coverage targets by functionality (>80% overall, >85% seasonal)
• Running tests (local, devtools, command-line)
• Coverage analysis instructions
• Detailed description of test-seasonal-models.R
• Test data generation utilities
• Best practices for writing new tests
• Continuous integration guidance
• Troubleshooting common testing issues

Updated .Rbuildignore

Added exclusions for development/testing artifacts:

• run_coverage_analysis.R (dev script)
• coverage_report.html (generated report)
• coverage_data.rds (coverage data)
• tests/TESTING_GUIDE.md (developer docs)

Impact on CRAN Readiness

This commit addresses three critical recommendations from the readiness analysis:

1. ⚠️ -> ✅ Added comprehensive tests for sar_pmm2(), sma_pmm2(), sarma_pmm2(), sarima_pmm2()

2. ⚠️ -> ✅ Added tests for S4 classes SARPMM2, SMAPMM2, SARMAPMM2, SARIMAPMM2

3. ⚠️ -> ✅ Implemented covr-based coverage analysis infrastructure

Expected Coverage Improvement:

• Before: ~357 lines of tests, estimated 70-75% coverage
• After: ~850+ lines of tests, target >80% overall, >85% seasonal

These additions significantly strengthen the package's test suite and bring it in line with CRAN quality standards for robust R packages.

Testing on real hardware with R installed is required to verify coverage metrics and ensure all tests pass.