fix(lib): stop collapsing getStreamAddress failures into 'stream not …

[Tobi-8]

Summary

• getStreamAddress no longer collapses every failure into null.
• null is returned only for scvVoid (factory says the stream ID does not exist).
• RPC/network/timeout errors and a missing NEXT_PUBLIC_FACTORY_CONTRACT_ID
  (already raised loudly by isMock()) now propagate, so /stream/[id] shows
  the real error instead of a misleading "Stream not found" for an existing stream.
• Updated the lib/stream.test.ts case that asserted the old swallow-to-null
  behavior, and added a page-level regression test asserting the error surfaces.

Testing

• lib/stream.test.ts: getStreamAddress address-lookup case updated to assert rejection; passing.
• app/stream/[id]/__tests__/page.test.tsx: new test verifying an address-lookup rejection renders the error, not "Stream not found"; all 7 tests pass.
• ESLint clean on changed files.

Closes #383