test: add jest config, CI, and repair the suite

[janakiram7755]

Stacked on #2. The repo had no jest config and no CI — both lived at the old monorepo root.

9/9 suites, 62/62 tests green.

Jest config follows upstream: ts-jest compiles to CommonJS for the test run while tsc builds lib/ as ESM. ESM namespaces are immutable so jest.spyOn cannot patch them; this keeps all 80 existing spyOn calls working with no restructuring. Same moduleNameMapper + transformIgnorePatterns approach they use.

Repaired tests, all of which were asserting upstream's old contract:

• getCacheEntry(key, restoreKeys, paths, opts) — ours takes 4 args, not 3
• reserveCache(key, chunks, version) — ours is positional, not an options object
• downloadCache(provider, ...) — ours dispatches on provider; replaced the URL-based Azure/http-client tests with s3 / gcs-without-token / unknown-provider
• ArtifactCacheEntry / ReserveCacheResponse no longer exist; fixtures now use the real CommonsGetCacheResponse / CommonsReserveCacheResponse
• isFeatureAvailable reads WARPBUILD_RUNNER_VERIFICATION_TOKEN, not the cache URL
• the getCacheVersion expectations were correct all along but omitted the 4th argument, which left process.arch in the hash and made them machine-dependent

Ref: actions/toolkit jest.config.js