fix: @inject works on HTTPEndpoint / WebSocketEndpoint methods

[lesnik512]

Closes #18.

What was wrong

@inject returned async def wrapper(connection). Bound as a class attribute, Python passes self first, so the wrapper received two positional arguments and raised on the first request, before any DI ran:

TypeError: Ep.get() takes 1 positional argument but 2 were given

The issue describes an AttributeError on connection.scope; that is not what happens. The arity check fails first. Either way it is a request-time error with nothing in the message that points at @inject, and with the scope key private (docs/adr/0002) there was no manual fallback.

Decision

One @inject for functions and methods. The wrapper is now async def wrapper(*args, **kwargs): it locates the connection as the first positional Request or WebSocket, resolves the markers from that connection's child container, and calls the handler with everything it was given plus the resolved values. A handler that receives no connection fails with a TypeError naming the handler.

Rejected:

• A sibling inject_method decorator. Starlette's own hook shapes already differ (on_connect(ws), on_receive(ws, data), on_disconnect(ws, close_code)), so a method-specific decorator would still have to forward trailing arguments, at which point it is the general decorator with a second name. Flask and aiogram in this org already wrap with *args, **kwargs.
• A TypeError at decoration time. A function in a class body is an ordinary function when the decorator sees it; the only signal is a first parameter named self, which is a convention, not a fact.

Finding the connection by isinstance rather than by position is what makes the three WebSocket hooks work without special-casing. on_disconnect is safe to inject into: Starlette calls it inside dispatch's finally, before the ASGI app returns and the middleware deletes the scope entry.

FastAPI and Litestar have no analogue: neither has a class-based path outside their native Depends / Provide. This is Starlette-only.

Tests

• tests/test_endpoints.py: HTTPEndpoint.get resolving APP- and REQUEST-scoped providers plus the Request context provider; WebSocketEndpoint.on_connect, on_receive and on_disconnect resolving SESSION-scoped providers plus the WebSocket context provider; the no-connection TypeError. The first two were red before the fix with the error above.
• test_finished_request_leaves_no_cyclic_garbage is parametrized over a function route and an HTTPEndpoint route, so the no-cyclic-garbage invariant is shown to hold on the new path rather than assumed.

Coverage stays at 100%.

Docs

• README: inject row in the API table, plus a class-based example under Usage.
• docs/adr/0002: no longer describes class-based endpoints as an open gap. Both ADRs are cut to one paragraph, per the org standard for docs/adr/.

Out of scope

• Synchronous def get on an HTTPEndpoint. inject is async-only for function endpoints today; unchanged.
• The Starlette page on the modern-di docs site has no class-based section. That is a separate PR in modern-di.

Release

Minor bump: the public inject contract widens.