refactor: single-source connection->scope mapping

[lesnik512]

What

Single-sources the connection→scope mapping in build_di_container: instead of hardcoding Scope.REQUEST / Scope.SESSION, it derives each child container's scope from the providers (fastapi_request_provider.scope / fastapi_websocket_provider.scope). The Request↔REQUEST / WebSocket↔SESSION pairing now lives in exactly one place — the ContextProvider definitions.

The scope: Scope | None = None annotation becomes a plain scope = None (inferred IntEnum | None), since AbstractProvider.scope is statically IntEnum.

Why

The scope a connection's child container is built at must match the scope its ContextProvider reads from, but the pairing was written twice with nothing enforcing agreement — a latent drift hazard. Deriving the scope from the provider makes them agree by construction.

This mirrors the same change just shipped in modern-di-litestar (#22) — the two integrations carry the identical pattern. (modern-di-faststream already eliminated the analogous writer/reader drift by naming its shared container keys.)

Behavior

Unchanged. The if Request / elif WebSocket structure and the scope=None default for any other connection are preserved exactly — only the scope source changes. The ContextProvider definitions are untouched.

Checks

• just lint-ci — clean (ruff, ty)
• just test-ci — 7 passed, 100% line coverage

Net 0 lines (3 insertions, 3 deletions).