Conformance client: legacy-lifecycle scenarios fail with -32020 missing MCP-Protocol-Version header at 2026-07-28

[DaleSeo]

Several 2026-07-28 client scenarios drive the regular streamable HTTP client through the legacy initialize lifecycle. The suite's mock server requires the MCP-Protocol-Version header on every POST at this spec version, and the legacy lifecycle does not send it before negotiation completes, so the run dies at initialize:

{"jsonrpc":"2.0","id":0,"error":{"code":-32020,"message":"Missing MCP-Protocol-Version header"}}

Affected scenarios (verified at current main 2e2c791; identical at pre-#973 839922d, so not a regression from the SEP-2575 merges):

• tools_call — 0/1 (passed in the 2026-07-13 baseline in Tracking: 2026-07-28 spec conformance  #977)
• auth/scope-step-up — 11/13; dies before the escalation round, so scope-step-up-escalation and sep-2350-scope-union-on-reauth never run. This means the suite cannot currently verify feat: accumulate client-side scopes during step-up authorization (SEP-2350) #888 (SEP-2350, merged 2026-07-16).
• http-standard-headers — 4/5; fails sep-2243-client-includes-standard-headers ("Missing Mcp-Method header on initialize request")

Likely fix: switch these scenario paths in conformance/src/bin/client.rs to the modern discover lifecycle added by #995 (ClientLifecycleMode::Discover / Auto) when running at 2026-07-28, which sends per-request metadata and the version header on every request. request-metadata already passes 5/5 through that path.

Repro:

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

Part of #977. Related: #978 (client-side SEP-2243 header work).