test: close the empty-batch hole in the e2e unstubbed-request guard (closes #187)

The guard that reports unrecognised POST bodies used batch.every(), which is
vacuously true on an empty array, so a POST with body [] was answered 200 []
and escaped the one mechanism whose job is to make unrecognised outbound
traffic fail the suite rather than pass silently. Unreachable in practice
today, which is exactly the qualifier that stops being true later.

The comment explaining the guard also described a mechanism that does not
exist: playwright-core decodes a binary body lossily rather than returning
null, so such a body reaches the JSON parse as mojibake and is reported by the
catch, while only an absent or empty body decodes to null and is reported by
the type guard. Both are reported; the comment now describes the two real
routes.

TODO.md

@@ -44,6 +44,14 @@ undefined identifiers, which is how
 
 # Completed Steps
 
+- 2026-08-12: Closed the empty-array hole in the end-to-end unstubbed-request
+  guard. `batch.every()` is vacuously true on `[]`, so a POST with body `[]` was
+  answered `200 []` instead of failing the suite; the guard now rejects an empty
+  batch, demonstrated green-before/red-after with a throwaway probe. The comment
+  claiming `postData()` returns `null` for undecodable bodies was corrected to
+  the two real paths — an absent or empty body decodes to `null`, a binary body
+  decodes lossily into invalid JSON
+  ([#187](https://git.eeqj.de/sneak/AutistMask/issues/187)).
 - 2026-08-12: The transaction confirmation screen has browser coverage. The
   end-to-end suite reaches ConfirmTx for both the native ETH and the ERC-20 path
   off a funded-balance fixture, and asserts the pending, funded, over-balance