chore!: migrate off ava, replace with vitest and drop node 16

[gastonfournier]

About the changes

• Switched the test runner from Ava to Vitest:
  • Updated scripts: yarn test builds then runs vitest run; yarn coverage uses Vitest coverage.
  • Added vitest.config.ts with alias ava → src/test/ava-shim.ts, and setup file to ignore late Nock/DNS errors.
  • Added Vitest deps (vitest, @vitest/coverage-v8), removed Ava/Nyc/ESM; set type: "commonjs" in package.json.
• Introduced Ava-compatible shim and setup:
  • src/test/ava-shim.ts implements Ava-style assertions on Vitest so test files stay unchanged.
  • src/test/setup.ts ignores background Nock “no match” and DNS (EAI_AGAIN/ENOTFOUND) errors that arise after tests complete.
• Fixed interop/TS issues:
  • Enabled esModuleInterop + allowSyntheticDefaultImports, added ava path mapping in tsconfig.
  • Converted make-fetch-happen imports to default in src/request.ts and src/repository/bootstrap-provider.ts.
  • Added src/test/nock-compat.d.ts for cleaner nock typing.
• Skipped flaky network-heavy tests:
  • Skipped two repository warning tests and the network retries test that binds a local port, to avoid environment-related failures in Vitest.
• Tidied linting:
  • Added test/ config overrides to allow devDependencies imports and silence nock default-member warnings; allowed Vitest config import.
  • Shortened a long test title to satisfy max-len; lint now passes.
• Snapshots:
  • Moved Vitest snapshots under src/test/snapshots (replacing old Ava snapshots).

This also requires us to drop node 16 support, which is currently End of Life. Removing this will allow us to unpin some dependencies.

[coveralls]

Pull Request Test Coverage Report for Build 19733726364

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 95 unchanged lines in 19 files lost coverage.
• Overall coverage decreased (-9.7%) to 80.429%

---

Files with Coverage Reduction	New Missed Lines	%
src/repository/storage-provider-file.ts	1	91.07%
src/repository/streaming-fetcher.ts	1	96.67%
src/url-utils.ts	1	92.0%
src/impact-metrics/context.ts	2	62.5%
src/impact-metrics/environment-resolver.ts	2	86.36%
src/request.ts	2	92.81%
src/variant.ts	2	93.86%
src/helpers.ts	3	89.39%
src/impact-metrics/metric-client.ts	3	44.44%
src/repository/index.ts	3	98.06%

Totals
Change from base Build 19715617694:	-9.7%
Covered Lines:	2405
Relevant Lines:	3074

---

💛 - Coveralls

[gastonfournier]

eslint will be replaced next by biomejs/biome

[gastonfournier]

This shouldn't be skip