test: setup code coverage

[MridulTailor]

Closes #789

Summary

Sets up code coverage reporting using @vitest/coverage-istanbul.

@vitest/coverage-v8 was attempted first but is incompatible with the Cloudflare Workers Vitest pool, it depends on node:inspector which is unavailable in that runtime. Istanbul is the supported alternative per the Cloudflare docs.

Changes

• package.json: adds test:coverage script
• vitest.config.ts: enables Istanbul coverage with text/lcov reporters and modest line/function thresholds
• .github/workflows/test.yml: CI now runs test:coverage
• .gitignore: adds coverage/ to prevent generated artifacts from being committed
• package-lock.json: updated for @vitest/coverage-istanbul@3.2.4

[cursor]

PR Summary

Low Risk
Low risk: this is limited to test/CI configuration and devDependencies, with no production runtime behavior changes. Main risk is CI failures if coverage thresholds or include/exclude patterns don’t match current test suite.

Overview
Adds code coverage reporting for Vitest using @vitest/coverage-istanbul, including a new test:coverage script and coverage configuration (reporters, include/exclude, and 80% line/function thresholds) in vitest.config.ts.

Updates CI (.github/workflows/test.yml) to run test:coverage instead of test, and ignores generated coverage/ artifacts via .gitignore; lockfile is updated accordingly.

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[avivkeller]

We should upload to codecov, no?

[MridulTailor]

Sure, I can add that. Just want to check, does the repo already have Codecov enabled on the org side?

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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