Fix MARVIS MCP startup handshake

[fangsylar-pixel]

Summary

• restore auto-detection of newline-delimited JSON-RPC used by MARVIS
• retain Content-Length framing for standard MCP clients
• add a MARVIS framing regression test and changelog entry

Verification

• 32 Python tests passed
• extension JavaScript syntax check passed
• runtime smoke test passed
• real MARVIS-style initialize handshake passed using the configured py.exe command