[Bug]: ag-ui-langgraph __init__ eagerly imports fastapi, breaking middleware-only (non-web) installs

### Pre-flight Checklist

- [x] I have searched [existing issues](https://github.com/ag-ui-protocol/ag-ui/issues) and this hasn't been reported yet.
- [x] I am using the **latest** version of AG-UI (`ag-ui-langgraph` 0.0.42, also reproduces on `main`).

### Describe the Bug

`fastapi` is declared as an **optional** dependency in the `ag-ui-langgraph` `pyproject.toml`:

```toml
[project.optional-dependencies]
fastapi = ["fastapi>=0.115.12"]
```

…but the package's top-level `__init__.py` unconditionally imports the FastAPI endpoint module:

```python
# ag_ui_langgraph/__init__.py
from .endpoint import add_langgraph_fastapi_endpoint
```

and `endpoint.py` imports FastAPI at module top:

```python
# ag_ui_langgraph/endpoint.py
from fastapi import FastAPI, HTTPException, Request
from fastapi.responses import StreamingResponse
```

Because Python evaluates a package's `__init__.py` before any submodule, **any** import from the package — including `from ag_ui_langgraph.middlewares.state_streaming import StateStreamingMiddleware` — fails on a default install (`pip install ag-ui-langgraph`, i.e. without the `[fastapi]` extra). The "optional" dependency is effectively mandatory for every consumer, so middleware-only / non-web consumers cannot import anything without pulling in `fastapi`/`starlette`/`uvicorn`.

### Steps to Reproduce

```bash
python -m venv .venv && source .venv/bin/activate
pip install ag-ui-langgraph # default install, NO [fastapi] extra
python -c "from ag_ui_langgraph.middlewares.state_streaming import StateStreamingMiddleware"
```

### Expected Behavior

Middleware-only consumers (e.g. a LangGraph backend that is not a web server) should be able to `import ag_ui_langgraph` and its middleware/utils submodules without installing `fastapi`. FastAPI should only be required when actually using `add_langgraph_fastapi_endpoint`, consistent with it being an optional extra.

### Environment

```
ag-ui-langgraph 0.0.42 (also reproduces on main)
Python 3.10–3.12
fastapi NOT installed (default install, no [fastapi] extra)
```

### Logs & Errors

```shell
ModuleNotFoundError: No module named 'fastapi'
File ".../ag_ui_langgraph/__init__.py", in
from .endpoint import add_langgraph_fastapi_endpoint
File ".../ag_ui_langgraph/endpoint.py", line 1, in
from fastapi import FastAPI, HTTPException, Request
```

### Additional Context — suggested fixes (any one resolves it)

1. **Lazy export (smallest change):** drop the eager `from .endpoint import ...` from `__init__.py` and expose it via module-level `__getattr__` (PEP 562) so `add_langgraph_fastapi_endpoint` only triggers the fastapi import when accessed.
2. **Guarded import:** wrap the endpoint import in `try/except ImportError` in `__init__.py` and raise a clear "install `ag-ui-langgraph[fastapi]`" message only on use.
3. **Keep endpoint out of the default surface:** have consumers do `from ag_ui_langgraph.endpoint import add_langgraph_fastapi_endpoint` and stop re-exporting it from the top-level `__init__.py`.

Option 1 (`__getattr__`) preserves the current public API while keeping fastapi truly optional.

**Workaround today:** consumers must either install the unwanted `[fastapi]` extra (pulls in `fastapi`/`starlette`/`uvicorn`) or inline the middleware. We currently inline `StateStreamingMiddleware` to avoid bolting a web stack onto a non-web backend.

Happy to open a PR for option 1 if a maintainer can confirm the preferred approach.