Conformance client: auth scenarios unmapped since #991 dispatch change (blocks verifying SEP-2468/SEP-2352 fixes)

[DaleSeo]

#991 replaced the conformance client's unknown-scenario fallback (run_auth_client) with anyhow::bail!("Unsupported conformance scenario"). Ten client scenarios in the 2026-07-28 suite relied on that fallback and now fail immediately:

• auth/iss-supported, auth/iss-not-advertised, auth/iss-supported-missing, auth/iss-wrong-issuer, auth/iss-unexpected, auth/iss-normalized
• auth/offline-access-scope, auth/offline-access-not-supported
• auth/authorization-server-migration (Implement SEP-2352: Clarify authorization server binding and migration #879)
• auth/metadata-issuer-mismatch (SEP-2468 follow-up: validate issuer in authorization server metadata #983)
• json-schema-ref-no-deref — chore: update client conformance test scenarios #991 also mapped this to run_discover_client, which hangs (30s timeout) against the scenario's mock server; it passed 1/1 at the 2026-07-13 baseline

All of these passed (or partially passed) in the 2026-07-13 baseline in #977 via the fallback. Client stderr:

Error: Unsupported conformance scenario: auth/iss-supported

Notably this means the suite cannot currently verify #996 (SEP-2468 issuer validation, merged 2026-07-16) — auth/metadata-issuer-mismatch errors out before exercising the flow — nor future SEP-2352 work for #879.

Fix: add explicit dispatch arms in conformance/src/bin/client.rs for these scenario names (most can map to run_auth_client; authorization-server-migration needs its own flow once #879 lands).

Repro:

cargo build -p mcp-conformance
npx -y @modelcontextprotocol/conformance@0.2.0-alpha.9 client \
  --command ./target/debug/conformance-client \
  --scenario auth/metadata-issuer-mismatch --spec-version 2026-07-28

Part of #977.