docs: adapters/README MongoDB parity cleanup

[techiejd]

Fixes pre-existing drift in the contract README (adapters/README.md). MongoDB shipped as an adapter, but this doc's quick-links and feature-parity table were never fully updated — and the parity table made a false claim about Cloudflare.

Changes (docs-only, independent of the findEmbeddingsByIds work)

• Quick-links: add the MongoDB package link alongside pg and cf.
• Feature parity table: add a MongoDB column with verified values:
  • Real-time + bulk ingest ✅
  • where operators: full (passes its own vectorSearchWhere.spec.ts, 31 tests)
  • like regex ✅ (has escapeRegExp)
  • Migration CLI bin ❌ — search indexes created at runtime via ensureSearchIndex (no bin)
  • Score range: vectorSearchScore, range depends on the index similarity metric (no fixed range asserted)
• Fix the false CF claim: the table said "Both pass vectorSearchWhere.spec.ts" — but CF has no such file; its where coverage lives in where.spec.ts (59 tests). Now stated accurately per-adapter.

All test counts (PG 31 · MongoDB 31 · CF 59) and the absence of a MongoDB bin were verified against the live tree. No code or test files touched.