Add url_for method to Router for named route URL generation

[Copilot]

Router had no way to generate URLs by route name, making reverse URL lookup impossible without manually constructing paths.

Changes

• RouteNotFound exception — raised when url_for is called with an unknown name; exported from blacksheep
• Route.url(**params) — builds a URL from the route pattern by substituting named parameters
• name parameter on all registration methods — optional name kwarg added to add(), add_get/post/...(), and all HTTP method decorators (get, post, etc.) on RouterBase
• Router.url_for(name, **params) — resolves a named route (including across sub-routers) and returns its URL with parameters substituted; raises RouteNotFound if the name is not registered
• Router._named_routes — internal dict[str, Route] populated at add() time, cleared on reset()

Usage

router = Router()

@router.get("/users/:username", name="user_detail")
def user_detail(): ...

router.apply_routes()

router.url_for("user_detail", username="johndoe")  # "/users/johndoe"

# Sub-routers are also searched
sub = Router()

@sub.get("/items/{int:item_id}", name="get_item")
def get_item(): ...

parent = Router(sub_routers=[sub])
parent.url_for("get_item", item_id="42")  # "/items/42"

Original prompt

Work on #256

Created from Copilot CLI via the copilot delegate command.

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[RobertoPrevato]

The url_for method must be added to request to support base_path.