feat: add conformance scenarios for 2026-07-28 stream cancellation and per-request logLevel

[elang2]

Summary

Adds two new conformance scenarios targeting the MCP 2026-07-28 specification:

• HTTP Stream-Close Cancellation — validates that closing the SSE response stream acts as request cancellation (replacing notifications/cancelled on HTTP transport)
• Per-Request LogLevel — validates _meta.io.modelcontextprotocol/logLevel filtering, scoping, and invalid value rejection

Both scenarios use introducedIn: DRAFT_PROTOCOL_VERSION and run only under --suite draft or --spec-version 2026-07-28, excluded from default CI until the spec is published as a dated release.

Scenarios Added

http-stream-cancellation (3 checks)

1. Stream close = cancellation — server handles mid-flight SSE disconnect gracefully
2. Server health after cancellation — server remains operational after client aborts
3. Rapid burst stability — 5 rapid open+close cycles don't crash/deadlock the server

per-request-loglevel (4 checks)

1. Positive emission — notifications/message emitted when logLevel is set (WARNING if server opts out)
2. Severity filtering — MUST NOT deliver messages below requested level
3. Invalid logLevel rejection — SHOULD respond with -32602 for unrecognized values
4. Request-scoped delivery — logs MUST appear only on the requesting stream, not concurrent ones

Design Decisions

• Uses raw fetch with AbortController (not SDK client) to precisely control stream lifecycle
• Tracks serverEmitsLogs flag to SKIP filtering/scoping checks when server never emits (avoids false positives on servers that don't implement logging)
• Per-iteration timeouts in burst loop prevent hangs if server is overwhelmed
• Check 4 aborts the logging request's stream before consuming to prevent indefinite SSE hang

Test plan

• TypeScript compiles cleanly (npx tsc --noEmit)
• ESLint passes
• Scenarios correctly filtered into draftClientScenariosList via introducedIn check
• Validated by 6 independent review agents (spec compliance, code quality, hang prevention, false-positive analysis)

Spec References

• Cancellation (2026-07-28)
• Streamable HTTP Cancellation
• Logging (2026-07-28)