test(registry): drive discoverAll's paging loop with a fake server

[0dillon]

Summary

discoverAll pages through the registry at PAGE_LIMIT = 50 per call. It is covered only by a
live check against a registry holding a single entry, which returns on the first page and never
exercises the loop.

This is the same untested-loop shape as the readTtl chunking issue, and the two should be picked
up together — a fake server written for one is most of the fake server needed for the other.

The paging loop is worth pinning down because a bug in it is silent: a keeper that stops after the
first page maintains the first 50 contracts and never mentions the rest.

Acceptance Criteria

• A fake server returns several pages and a test asserts every entry is discovered exactly once.
• A test covers a total that is an exact multiple of PAGE_LIMIT, the boundary where a naive
  loop either stops early or makes one empty request too many.
• A test covers an empty registry.
• A test asserts the loop terminates rather than paging forever when the registry reports a
  count that disagrees with what page returns.
• The bigint branch of requireNumber is covered, since count can decode either way.

Tech Stack

TypeScript 5.7 on Node 22+ (CI runs 22 and 24), ESM with moduleResolution: "NodeNext" — relative
imports need the .js extension. @stellar/stellar-sdk is pinned to an exact version; verify RPC
API shape against the installed copy, not against documentation. Tests are Vitest under test/ and
run without a network. npm run format:check, npm run lint, npm run build and npm test must
all be clean.