Reconcile the emitted-code seam-pin assertion convention with the testing doc

The development principles ban code-string assertions on generated method bodies, yet the accepted emission-pin convention (R45’s TenantRoutedFetcherPipelineTest, TenantRuntimeKeyTypeTest, R46’s TenantFanOutFetcherPipelineTest) asserts TypeSpec.toString() fragments that include call-site shapes inside method bodies, on the argument that the pinned strings are the seams (carrier statics, factory keys, routing calls) rather than incidental body text. The R46 Done-gate review flagged the tension: either the testing doc gains a paragraph legitimising seam pins and drawing the line against incidental-body pins (and the existing tests get audited against that line), or the pins migrate to structural TypeSpec assertions. Decide once, write it down, and align the three test classes; today each new reviewer re-litigates the convention.