feat: filter vehicles by synthetic device connection

[elffjs]

What

Adds a connection field to VehiclesFilter so callers can list all vehicles paired with a synthetic device that belongs to a given connection, keyed by the connection's address:

{
  vehicles(first: 100, filterBy: { connection: "0x1111...1111" }) {
    totalCount
    nodes { tokenId owner syntheticDevice { tokenId } }
  }
}

Because it's a filter on the existing vehicles query, it composes with the other filters for free — e.g. filterBy: { connection: "0x...", make: "Toyota", owner: "0x..." }.

How

A vehicle links to a connection only through its synthetic device, and synthetic_devices.connection_id is a FK to connections.id (the 32-byte padded name), not the address. So when the filter is set, queryModsFromFilters joins vehicles → synthetic_devices → connections and filters on connections.address. No migration required.

Tests

Added connection + synthetic-device fixtures to the vehicle filter suite and three cases: connection alone, connection + owner, and a no-results combo. Full internal/repositories/vehicle package suite passes.

Notes / open questions

• Naming: went with the plain connection on the filter. syntheticDeviceConnection would be more self-documenting but verbose — happy to rename.
• MCP: did not add an @mcpExample demonstrating the connection filter on the vehicles query. Can add one if we want the MCP tool to advertise it.

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