[tamnd]

Part of #2492.

PostgreSQL resolves a string literal in an aggregate call through the usual function lookup. The literal has the unknown type, and the aggregate candidates decide which type it gets. So avg('4') has two equal candidates and fails with 42725, and max('x') reads the literal as text.

The first part of this work landed in #3002 by accident, because a commit chain picked up the edit. That part resolves the unknown arguments of a plain aggregate call. This PR finishes the work:

• A windowed aggregate call now resolves its unknown arguments too, so sum('x') over () fails with 42725 as in PostgreSQL.
• regr_r2 kept a NaN answer as 1, because f64::min drops NaN. It now clamps only a real value above 1, so a NaN input gives NaN as in PostgreSQL.
• A server test covers both paths and the errors.

The harness goes from 1869 to 1873 statements that are the same on rudb, with no new differences.