feat: send X-Weaviate-Client-Integration telemetry header (#275)

## What this does

Weaviate recently added server-side telemetry that tracks **client
integration** usage via a new HTTP/gRPC header
(weaviate/weaviate#10535):

```
X-Weaviate-Client-Integration: langchain/<version>
```

The server parses this as `name/version`, aggregates it into
`clientIntegrationUsage`, and reports it alongside the existing
`X-Weaviate-Client` header. Today `langchain-weaviate` sends no such
header, so LangChain usage is invisible in Weaviate's telemetry.

This PR makes `WeaviateVectorStore` tag its connection with
`langchain/<version>` so adoption shows up in Weaviate telemetry.

## How

- The header is registered in `WeaviateVectorStore.__init__` via the
weaviate-client **public** integrations API
(`client.integrations.configure(...)`), which applies it to **both HTTP
and gRPC** transports on the already-connected client. All construction
paths (including `from_texts`) funnel through `__init__`.
- Registration is **best-effort**: the integrations import, config
class, and call are wrapped in a `try/except` that logs at debug and
never raises — so an older `weaviate-client` without that API, or any
failure, won't break construction.
- The reported version is the `langchain-weaviate` package version, read
at runtime via `importlib.metadata` with an `"unknown"` fallback. This
mirrors the client header (the version of the code making the call).

### On naming
The integration name is `langchain` (not `langchain-python`). The
language is already disambiguated downstream by the co-located
`X-Weaviate-Client` header (`weaviate-client-python` vs the TS client),
so a language suffix would be redundant, and this keeps parity with the
TS LangChain integration.

## Tests

Added unit tests (MagicMock-based, no Docker) covering: the exact header
value registered, that a registration failure does not break `__init__`,
and the version `"unknown"` fallback. Full suite passes locally (66
tests) with coverage at 99.65% (gate 98.06%); ruff and mypy clean.

## Notes for reviewers

- The version bump commit takes the package `0.0.6 -> 0.0.7`. Feel free
to drop it if you'd rather bump as part of your own release flow — the
header picks up whatever version is installed at runtime.

---

## What's new in 0.0.7 (since 0.0.6)

### Features
- **Weaviate client-integration telemetry** — sends the
`X-Weaviate-Client-Integration: langchain/<version>` header so LangChain
usage shows up in Weaviate telemetry (this release's headline change). —
#275
- **Custom schema support** — `WeaviateVectorStore` now accepts a
user-provided schema instead of always generating the default. — #254
- **simsimd made an optional dependency** — vector distance computation
falls back automatically to NumPy/SciPy when `simsimd` isn't installed,
lightening the default install. — #249

### Tooling / maintenance
- **CodeQL + higher coverage gate** — added CodeQL scanning and raised
the test-coverage requirement; version bumps. — #253
- **Docs reference fixes.** — #248

### Dependency & security updates
~18 Dependabot bumps (PRs #250, #256–#274), including security-relevant
ones: `cryptography`, `urllib3`, `authlib`, `tornado`, `idna`,
`requests`, `orjson`, `protobuf`, `langsmith`.

---------

Co-authored-by: Mason Daugherty <mason@langchain.dev>