Live-verify the 9 server/managed backend adapters (deferred from #11)

[thorwhalen]

Deferred item from #11: the 9 backend adapters written correct-by-construction
but never run against real instances — sqlite_vec, pgvector, pinecone,
weaviate, milvus, redis, elasticsearch, mongodb, turbopuffer.

Scope of this issue

Verify the 7 adapters reachable without a cloud account, against real engines:

• Docker containers: pgvector, redis, elasticsearch, weaviate, mongodb
  (mongodb-atlas-local).
• Embedded Milvus Lite: milvus (same adapter code path as a Milvus server;
  only the client constructor differs).
• Loadable-extension Python: sqlite_vec.

pinecone and turbopuffer remain deferred — they need cloud accounts.

Outcome

All 344 contract tests pass across 13 backends. 8 real bugs found and fixed;
see the PR. A reusable harness was added: tests/docker-compose.yml,
a reworked tests/conftest.py that TCP-probes and skips unreachable server
backends (CI stays green without them), and tests/README.md.

Still deferred (tracked in #11)

• pinecone, turbopuffer — need cloud accounts.
• SupportsHybrid implementations + client-side RRF fallback.
• AsyncClient / AsyncCollection parallel protocols.
• LangChain adapter shims; the F() filter builder.

Refs #11