Feature/issue 23 http factory

[jpsiddharth2008]

What this does

• After [mcp-2026] Remove JSON-RPC sidecar; one /mcp owner (official mcp 2.x) #22 removed the JSON-RPC sidecar, the official mcp SDK became the sole owner of /mcp, but four era-dependent behaviors were left disagreeing between the two engines that used to share it.
• resources.subscribe now comes from one source (protocol_era != "modern") shared by NitroStackMcpServer.get_capabilities(), server/discover, and the real _subscribe_resource handler — previously get_capabilities() hardcoded True and discover hardcoded False, both wrong for at least one era.
• /json/version now reports protocol_version_for_era(protocol_era) instead of a hardcoded 2025-06-18.
• CORS is down to one implementation point — Starlette's CORSMiddleware in build_http_app — and it now actually reads MCP_CORS_ALLOWED_ORIGINS instead of hardcoding allow_origins=["*"].
• stdio.py's modern era loop no longer imports the private mcp.server.runner._serve_modern_stream; rebuilt from public mcp.server.runner APIs (JSONRPCDispatcher, modern_on_request, Connection, ServerRunner, NotifyOnlyOutbound, aclose_shielded).
• Depends on [mcp-2026] Remove JSON-RPC sidecar; one /mcp owner #34 (branched from it; merge [mcp-2026] Remove JSON-RPC sidecar; one /mcp owner #34 first).

Test plan

• pytest tests/test_transport_http.py tests/test_lifecycle_http.py tests/test_mcp20_stdio.py tests/test_transports.py -q — the issue's own verify command → 31 passed
• pytest tests/test_mcp23_http_factory.py tests/test_mcp22_official_http_owner.py -v — new tests covering the issue's 5-point checklist, plus [mcp-2026] Remove JSON-RPC sidecar; one /mcp owner #34's regression tests → 15 passed
• Full suite → 561 passed, 0 failed (excluded items independently confirmed pre-existing on [mcp-2026] Remove JSON-RPC sidecar; one /mcp owner #34's base or flaky Windows socket tests, not regressions from this branch)
• Manual: boot examples/calculator_server.py with NITRO_MCP_PROTOCOL_VERSION=modern|auto|legacy and confirm GET /json/version and server/discover's resources.subscribe track the chosen era