test(uploads): pin the idle bound an unset App resolves to (#56)

Round-2 gatekeeper finding: production never sets UploadIdleTimeout, so it
always runs the zero-means-default branch, and nothing asserted what that
branch resolved to (`idle = time.Hour` kept the suite green).

Extract the resolution into App.uploadIdleTimeout(), expose it via
export_test.go, and assert (&web.App{}) resolves to exactly 60s, a
negative value too, and a set value is honoured. Verified red against the
time.Hour mutant before implementing.

Rename the socket test to what it observes:
...AnUnsetIdleTimeoutDoesNotCutABodyWithSubSecondGaps.

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

internal/web/export_test.go

@@ -1,5 +1,11 @@
 package web
 
+import "time"
+
 // 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
+
+// UploadIdleTimeoutForTest exposes the bound an App resolves to (#56), so a test can pin the
+// value the zero-value App -- production's -- actually uses rather than only the constant.
+func (a *App) UploadIdleTimeoutForTest() time.Duration { return a.uploadIdleTimeout() }