vMCP migration test-coverage follow-ups: pagination gate + conformance coverage

Follow-ups split out of the mcp-go → mcpcompat migration review (#5729) so they don't block that PR. Both are test-coverage gaps @jhrozek flagged as out-of-scope for the migration itself.

1. pagination_regression_test.go gates nothing while skipped

test/integration/vmcp/pagination_regression_test.go is t.Skip'd. Before un-skipping, confirm the test actually exercises a cursor loop: if CreateBackendServer returns all ~1100 tools in a single page, the GreaterOrEqual assertion passes without any pagination happening, and MCPClient.ListTools issues a single call with no cursor follow. Make the backend paginate at the 1000 boundary and drive the cursor loop, then un-skip.

2. MCP Conformance should exercise the swapped SDK

E2E Tests / MCP Conformance runs against thv run --transport streamable-http, served by the transparent reverse proxy (pkg/transport/proxy/transparent), which imports no mcpcompat/go-sdk code. The swapped SDK lives in vMCP and the stdio bridge, so a green conformance run says little about the migration. Point the conformance suite at a vMCP endpoint (and/or the stdio bridge) so it actually covers mcpcompat's protocol surface.

Context: stacklok/toolhive#5729 (part of epic #5743).

Start with test/integration/vmcp/pagination_regression_test.go and inspect CreateBackendServer and MCPClient.ListTools to verify the cursor loop before removing t.Skip. Then trace the E2E Tests / MCP Conformance entry point and the vMCP or stdio bridge endpoint. Done means pagination crosses the 1000-tool boundary and conformance exercises the swapped mcpcompat protocol surface.