test(uploads): pin the deployed idle bound and its unavailable warning (#56)

Gatekeeper review of PR #63 (comment 15139):
- the 60 s default is what production runs (cmd/tapir never sets
  UploadIdleTimeout) and no test covered it; 1 ns, 60 ms or 24 h all left
  the suite green. Pinned via export_test.go, plus a socket test with the
  timeout unset that accepts a body with 100 ms gaps (60 ms mutant red).
- the "upload idle bound unavailable" warning, the only signal that the
  control did not apply, is now asserted (removing it goes red).
- the deadline clear on io.EOF was redundant (net/http resets the
  deadline before its background read) and is removed; ADR-033 text
  corrected (the part can end before the body, so "cleared on clean
  EOF" did not hold).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/export_test.go

@@ -0,0 +1,5 @@
+package web
+
+// DefaultUploadIdleTimeout exposes the production idle bound (#56) to the black-box tests:
+// cmd/tapir never sets App.UploadIdleTimeout, so this constant IS the deployed value.
+const DefaultUploadIdleTimeout = defaultUploadIdleTimeout