[Conformance] Raise ResourceNotFoundError for unknown resource URIs

## Motivation and Context

Per SEP-2164, a `resources/read` request for a nonexistent resource answers with JSON-RPC error -32602 carrying
the requested URI in `error.data`, and MUST NOT return an empty `contents` array. The SDK supports this via
`MCP::Server::ResourceNotFoundError` (#402), but the conformance fixture's `resources_read_handler` returned
an empty array for unknown URIs, so the `sep-2164-resource-not-found` scenario of the 2026-07-28 conformance
requirements fails on the fixture ("Server returned a result with an empty contents array").

The gap never surfaced before because the scenario is part of the 2026-07-28 requirement set only;
the 2025-11-25 suite the fixture has been passing does not run it. The fixture now raises `ResourceNotFoundError`,
which is also the documented pattern for `resources_read_handler` users.

## How Has This Been Tested?

- `sep-2164-resource-not-found` passes 4/4 checks at `--spec-version 2026-07-28` against the fixture server.
- The `--requirements 2025-11-25` server leg passes 78/78, and each resources scenario reports no change,
  so known URIs are unaffected.
- RuboCop reports no offenses on the changed file.

## Breaking Changes

None. The change is confined to the conformance fixture; the shipped gem is untouched.

conformance/server.rb

@@ -577,7 +577,9 @@ module Conformance
             ).to_h,
           ]
         else
-          []
+          # Per SEP-2164, an unknown URI answers with -32602 carrying the URI in `error.data`;
+          # an empty `contents` array is forbidden.
+          raise MCP::Server::ResourceNotFoundError.new(uri, params)
         end
       end
     end