feat(client): cover empty and opaque pagination cursors - #583

An MCP client that stops on a falsy nextCursor can pass the existing non-paginated tools_call scenario while missing tools on later pages. Add client-pagination, following the proposal in #577, to exercise one complete tools/list traversal and a call to a tool discovered on its last page.

The three pages contain 1, 0, and 2 tools. They advertise an empty-string cursor, an opaque cursor containing whitespace, punctuation and Unicode, then no nextCursor. Four checks cover empty-cursor continuation, cursor preservation through an empty page, the scenario-completion gate, and end-of-results handling. Missing prerequisites produce explicit untestable failures/warnings. The termination SHOULD uses WARNING; a new discovery after the requested tool call is allowed. An invalid cursor followed by a successful retry still fails cursor preservation.

Register the scenario in core, extend everything-client, and add a deliberately broken client with six negative controls and a rediscovery positive control. Reuse the existing scenario context, transports, CLI runner and check types. No dependencies, shared runner changes or frozen requirements manifests are included.

Validation on Windows, Node 22.14.0, locked SDK 1.29.0, based on 96a6f94:

• npm run build; npm run check; npm test: 49 files, 659 tests passed, including 18 pagination tests.
• Built CLI: correct loop and post-task rediscovery pass at both 2025-11-25 and 2026-07-28; truthy-cursor, trim-cursor, recovered trim, empty-page, terminal overrun and inert controls fail as expected (16 runs). All controls that send messages pass wire-schema validation.
• The real TypeScript SDK is used for the 2025 stateful path; the existing stateless transport is used for 2026. This does not claim validation against every current SDK.
• Inverting the cursor-preservation condition makes the focused tests fail; the source was restored and the full suite then passed.

Reproduce after npm ci --ignore-scripts --no-audit --no-fund and npm run build:
node dist/index.js client --scenario client-pagination --spec-version 2025-11-25 --command "node --import tsx examples/clients/typescript/everything-client.ts"
node dist/index.js client --scenario client-pagination --spec-version 2025-11-25 --command "node --import tsx examples/clients/typescript/pagination-broken-client.ts"
Repeat with --spec-version 2026-07-28. The broken client's default is truthy-cursor. MCP_PAGINATION_DEFECT selects the other controls.

Related work: #520 mentions a three-page pagination prototype in its validation narrative; its public file list contains the external loader and trace-id example, rather than that pagination scenario. This PR implements built-in spec coverage and makes no claim to invent pagination testing. Core-suite placement and the older-spec applicability remain reviewable design choices; the explicit empty-string wording is in 2026, while the older schema permits empty strings and requires opaque cursors.

AI assistance: Codex prepared this implementation and ran the checks above. No independent human review is claimed.