feat(http): add stateless Streamable HTTP mode

[LevSky22]

Summary

• add opt-in MCP_HTTP_TRANSPORT_MODE=stateless while preserving the current stateful default
• use request-scoped MCP servers/transports for POST requests without issuing or retaining session IDs
• reject GET/DELETE on /mcp in stateless mode and document the lifecycle tradeoffs
• keep N8N_MODE independent so its client-compatible tool descriptions work with either transport lifecycle

Verification

• npm run typecheck
• npm run build
• focused HTTP transport suite: 55/55 passing
• focused N8N_MODE tool-description suite: 4/4 passing
• Docker protocol smoke: initialize, ping with a stale session header, tools/list, authenticated tool call, auth/tenant failures, GET/DELETE 405
• client smoke tests: Codex HTTP MCP, Hermes HTTP MCP (including repeated keepalives), and n8n 2.30.7 MCP Client Tool
• canary soak completed without post-cutover session-lifecycle errors

The broader unit run completed with 5,097 passing and 35 skipped; 186 database-dependent tests failed because the local sql.js build lacks FTS5 (no such module: fts5), unrelated to this transport change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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