Add test suite (89 tests) with vitest

Tests cover errors, HTTP client core, all 11 resource classes, and SSE
streaming. Runs in CI on Node 18/20/22 and before npm publish.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: realbub

.github/workflows/ci.yml

@@ -22,3 +22,4 @@ jobs:
       - run: npm ci
       - run: npm run typecheck
       - run: npm run build
+      - run: npm test

.github/workflows/release.yml

@@ -22,6 +22,7 @@ jobs:
 
       - run: npm ci
       - run: npm run build
+      - run: npm test
 
       - run: npm publish --provenance
         env: