Add backend validation for all canvas component types

[djbarnwal]

Implements backend validation for all 18 canvas component renderer types in ValidateRendererProperties.

• Validates required and optional fields for each renderer against the referenced metrics view (dimensions, measures)
• Groups cartesian charts (line_chart, bar_chart, area_chart, stacked_bar, stacked_bar_normalized) and circular charts (donut_chart, pie_chart) into shared validators
• Handles special cases: metrics_sql as an alternative to metrics_view, rill_measures/measures virtual color fields (type: value), plain-string color literals
• Adds helpers: requireMetricsView, getPathStringSlice, validateOptionalDimensionField, validateOptionalMeasureField, validateOptionalColorDimensionField
• Adds test functions covering valid specs, invalid field references, missing required fields, and edge cases

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[djbarnwal]

@begelundmuller The frontend canvas UI does not support metrics_sql as a renderer property for kpi or kpi_grid It is always configured with metrics_view.

However, the backend does handle metrics_sql as a generic renderer property: populateRendererRefs in reconcilers/canvas.go uses it to discover transitive metrics view references for security resolution, and canvases.go uses it during canvas resolution to extract referenced metrics views. Hence for now have added metrics_sql in the validation logic for those components.