auth/pre-registration: new pre-registration-auth conformance check fails #984

Part of #977.

On the 2026-07-28 suite (0.2.0-alpha.9), auth/pre-registration fails 5/6 with check pre-registration-auth. The scenario passed 13/13 on the stable 0.1.16 suite, so this is a check added in the alpha suite (the check set also changed shape, 13 → 6 checks).

Work: inspect the failing check's expectations (pre-registered client_id/client_secret supplied via MCP_CONFORMANCE_CONTEXT) and fix the auth flow in the SDK or the conformance client.

Repro:

cargo build -p mcp-conformance
npx -y @modelcontextprotocol/conformance@0.2.0-alpha.9 client \
  --command ./target/debug/conformance-client \
  --scenario auth/pre-registration -o results