Add scrollbar indicators for failing tests

[yogeshwaran-c]

Summary

• Adds overview ruler (scrollbar) markers for tests in a failed or errored state in the editor's gutter test decorations
• When a test fails, a red indicator now appears in the editor scrollbar at the line where the test is defined
• Uses the existing overviewRulerError theme color and OverviewRulerLane.Center positioning, consistent with how TestMessageDecoration already uses overview ruler indicators for error messages

This is a small, targeted change to createRunTestDecoration() in testingDecorations.ts. The TestMessageDecoration class already shows overview ruler indicators for test error messages, but the gutter decorations marking where tests are defined did not. This makes it easier to spot failing tests in large files without scrolling through the entire document.

Fixes #143203

Test plan

• Open a project with tests in a large file
• Run the tests so that some fail
• Verify that red indicators appear in the scrollbar/overview ruler at the lines where failing tests are defined
• Verify that passing/unset tests do NOT show scrollbar indicators
• Verify that the indicators use the standard error color from the theme (editorOverviewRuler.warningForeground)