feat: add test coverage visualization

[bookenjoyer67]

Adds client-side support for the deno/testCoverage LSP notification, enabling visual coverage feedback directly in the editor.

This creates a Run with Coverage profile in the Test Explorer. When tests finish, the extension receives coverage data from the LSP and:

• Highlights covered lines with a green background (using diffEditor.insertedTextBackground)
• Highlights uncovered lines with a red background (using diffEditor.removedTextBackground)
• Shows an overview in the ruler lane for quick navigation
• Displays a summary message with the average coverage percentage

The decorations update automatically when switching between editor tabs.

Changes:

• client/src/lsp_extensions.ts — New types: FileCoverage, CoverageNotificationParams, and the testCoverage notification
• client/src/testing.ts — Coverage handling in DenoTestController, reusing existing test run infrastructure

Requires: denoland/deno#36314

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