Conformance Track B: stateful HTTP/stdio modes, SEP-2549 absence-assert, remaining MRTR scenarios

[halter73]

Follow-up from #1610. Extensions to the cross-SDK conformance harness that were out of scope for the PR:

• Stateful HTTP + stdio conformance modes — the harness currently exercises draft stateless HTTP; add stateful-HTTP and stdio transport modes.
• Reinstate SEP-2549 absence-assert (upstream conformance PR Connecting to WebSocket based MCP Servers #310) — was dropped pending the RunContext refactor; restore once that lands.
• Two skipped MRTR scenarios (tampered-state HMAC validation, capability-check per-request gating) — ✅ Done in Default draft protocol support: sessionless + handshake-less (SEP-2575 + SEP-2567) #1610 (commit 52f97d4b). The est_input_required_result_tampered_state and est_input_required_result_capabilities ConformanceServer tools are implemented and both input-required-result-tampered-state / input-required-result-capability-check rows now run (14/14 RunMrtrConformanceTest pass against the local compat/conformance-draft build). In-process wire-level regressions added in MrtrProtocolTests.
• Upstream server/discover standalone scenario — file/track an upstream conformance-repo issue for the missing standalone scenario so we can cover the server/discover path end-to-end.

cc #1610