feat(GEN-4760): derive hosted MCP vidsheet schema

[mavxdegods]

GEN-4760

Makes the independently deployed hosted MCP server a thin consumer of the Rails-generated Vidsheet wire artifact.

What changed

• Vendors gen-backend-v2/docs/generated/vidsheet-operations-schema.json into the hosted MCP package and includes it in the deployment wheel.
• Adds fail-closed artifact parsing: missing, malformed, non-wire, markerless, or unmarked ordering-shaped contracts prevent unsafe tool publication.
• Derives spreadsheet_column.type and video_layer.type FastMCP enums directly from the artifact.
• Removes model-facing raw position inputs from column/layer create/update tools.
• Adds named gen_reorder_columns / gen_reorder_layers tools; they derive Rails' id_to_position wire payload internally.
• Layer creation reads the current cell and derives Rails' required append position internally.

Verification

• PYTHONPATH="$PWD/src" python -m pytest tests --tb=short — 12 passed
• Built a wheel and verified gen_mcp_server/contracts/vidsheet-operations-schema.json is included (627,286 bytes).
• Scoped Ruff clean for new contract/tests; existing unrelated Ruff findings remain in pre-existing scheduling code at server.py:1571 and server.py:1587.
• Scoped automated/manual review was run. The independent Claude review command could not execute because the shared Claude CLI weekly quota is exhausted; no reviewer findings were returned.

No merge or deploy performed.