test: exercise MaxBytesError branches with unknown-length bodies

WithMCPParse and WithScopeChallenge tests for oversized requests were
using strings.NewReader, which gives httptest.NewRequest a known
Content-Length. That let WithMaxBodySize reject the request in its
fast path before the request ever reached the middleware's own
io.ReadAll/isMaxBytesError handling, leaving those branches untested.

Reuse the existing unknownLengthBody helper (body_limit_test.go) so
these tests actually reach the fallback read path and cover the
*http.MaxBytesError handling added in WithMCPParse and
WithScopeChallenge.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>