fix: make built ESM entrypoint importable

[slegarraga]

Summary

• Add .js extensions to local relative source imports/exports so the built ESM files are directly importable by Node.
• Add a CI smoke check that imports ./dist/index.js after npm run build.

Why

The package is published with "type": "module", but the current build emits extensionless relative imports such as ./provider in dist/index.js. Node's ESM loader does not resolve those specifiers, so importing the built package entrypoint fails with ERR_MODULE_NOT_FOUND after a clean build.

The new CI check catches that failure mode before publishing:

node --input-type=module -e "await import('./dist/index.js')"

Validation

npm test
npm run build
node --input-type=module -e "await import('./dist/index.js'); console.log('entrypoint import ok')"
npm pack --dry-run

Local results:

• npm test: 3 files passed, 101 tests passed.
• npm run build: completed successfully.
• Entrypoint import: entrypoint import ok.
• npm pack --dry-run: tarball includes dist/index.js.