Add comprehensive tests with mock dependencies
Add tests that exercise the actual feature flows using fakes
for all external dependencies (OAuth, PDS, URL fetching).
New test files:
- tests/test-helpers.ts: Mock utilities for sessions and fetch
- tests/bookmarks.test.ts: 10 tests for bookmark CRUD
- tests/enrichment.test.ts: 11 tests for URL metadata extraction
- tests/plc-resolver.test.ts: 7 tests for DID resolution
Modified lib/session.ts to add setTestSessionProvider() hook
for injecting mock sessions in tests.
Total: 35 tests, all running without network calls.