fix(formatters): render warnings in markdown output

[slegarraga]

Summary

The markdown formatter never rendered warnings, even when --display-only-failures was not passed. Fixes #2928.

Root cause

markdown.ts filtered results with result.severity <= failSeverity. failSeverity defaults to error (DiagnosticSeverity.Error === 0), so only errors survived and warnings (severity 1) were dropped regardless of --display-only-failures.

failSeverity controls the process exit code, not what is displayed. The --display-only-failures filtering is already applied in the CLI before formatting (filterResultsBySeverity in commands/lint.ts), and the other formatters (stylish, text, json, pretty) render every result they receive.

Fix

Drop the redundant failSeverity filter from the markdown formatter so it renders all results it is given, consistent with the other formatters.

Test

Added a regression test asserting warnings are rendered with the default failSeverity of error. The existing markdown test continues to pass.

PASS @stoplight/spectral-formatters packages/formatters/src/__tests__/markdown.test.ts
  Markdown formatter
    ✓ should format as markdown table
    ✓ renders warnings regardless of failSeverity (#2928)