Add a combined screen for leveraged instruments

[LurigeLars]

This adds a read-only screen_leveraged_instruments tool for a common workflow that currently takes several MCP calls: finding certificates and warrants for the same underlying.

The tool takes a verified underlying order_book_id, a long/short direction, and optional product families. It:

• queries certificates and warrants concurrently
• paginates each family internally, with a hard cap of 200 results per family
• returns the fields that are useful for an initial comparison, such as issuer, leverage when available, bid/ask, spread, turnover, stop-loss and underlying identity
• keeps results from one product family if the other Avanza endpoint is temporarily unavailable

Prices and spreads are deliberately described as discovery data rather than execution quotes, since Avanza may return stale or missing values outside market hours.

I kept the change focused on the existing FastMCP 3.4.7 contracts. It does not change dependencies, transport, authentication or existing tool behavior.

Tests cover combined certificate/warrant collection, spread calculation, partial upstream failure, input validation and the registered MCP contract.