feat: implement automated reporting and visualization layer

[ayushman1210]

Description

Here's a breakdown of what I worked on:

• Updating the plotting functions: I refactored the old metric_timeseries_plot.R, metric_scatter_plot.R, and metric_residual_plot.R functions so they can easily consume the data.frame or data.table outputs straight from the run_benchmark() pipeline. I also took the opportunity to clean up the ggplot2 aesthetics (better themes, removing hardcoded date limitations, adding reference lines, etc.).
• Quarto report template: I created a new Validation_report.qmd template in inst/reports/. This generates a nice HTML scorecard (ILAMB-style) that compiles all our computed metrics alongside the plots.
• Reporting wrapper: I added generate_validation_report.R, which acts as a wrapper to render the Quarto template automatically at the end of a benchmark run.
• Testing: Added some basic tests in test-visualization.R to make sure the plots render correctly and the wrapper function handles missing templates gracefully.

Types of changes

• New feature (non-breaking change which adds functionality)

Fixes #4037