meterd+proxy: close the metering test gaps from the #247 review

Nobody has claimed this yet.

Two coverage gaps the review of #247 identified. Neither is a bug today; both are places where a silent regression would not be caught.

## The ErrorHandler wiring is untested

#247 added an `ErrorHandler` to the `ReverseProxy` that releases the reservation when the backend never produces a response. The release helper itself is tested (`TestReleaseReservationOnAbandonedRequest`), but nothing drives the actual proxy against a dead backend to prove the metering context value survives into the handler.

It does today: `ReverseProxy` builds `outreq := req.Clone(ctx)` from the incoming request, so the context carries through and the `meteringContextKey` lookup finds the info. That is a stdlib behavior we depend on and do not pin. A `httptest`-backed test that points a metered service at a closed port and asserts a usage report arrives would fix that.

## The new InvalidArgument paths are only tested one layer down

#247 made `GetPrice`, `ChallengeMinted` and `AuthorizeRequest` fail closed when the request model cannot be determined from the serialized body. That behavior is covered at the `modelFromRequestText` table level, but no `server_test.go` case asserts the RPC-level codes.

Worth pinning specifically because the guard's *placement* matters as much as its existence: the check in `AuthorizeRequest` sits before `s.store.Authorize`, which is what makes erroring safe (no reservation has been taken yet, so there is nothing to strand). A test asserting `codes.InvalidArgument` and an unchanged bundle balance would catch a future reordering.

## Acceptance

- A metered request to an unreachable backend produces a usage report carrying the echoed reservation.
- `GetPrice` / `ChallengeMinted` / `AuthorizeRequest` return `InvalidArgument` on an indeterminate model, and the bundle's reserved balance is unchanged afterwards.

Start with the ReverseProxy and its existing TestReleaseReservationOnAbandonedRequest, then add an httptest-backed case for a metered request to an unreachable backend and verify the usage report carries the echoed reservation. In server_test.go, cover GetPrice, ChallengeMinted, and AuthorizeRequest with an indeterminate model; done means each returns codes.InvalidArgument and the bundle's reserved balance is unchanged.

Written by the indexing model from the issue text.