fix: Allow named arguments to skip optional parameter positions

[sgrebnov]

Which issue does this PR close?

Related to this PR:

• Add PostgreSQL-style named arguments support for scalar functions apache/datafusion#18019

Fixes named argument resolution to follow PostgreSQL semantics where named arguments can target any parameter position without requiring intermediate optional positions to be filled.

Problem

Before this fix, resolve_function_arguments required positions 0..N to be filled when N arguments were provided. This incorrectly rejected valid PostgreSQL-style calls:

-- Signature: func(a, b DEFAULT 10, c DEFAULT 20, d DEFAULT 30)
SELECT func(1, d => 40);  -- Valid in PostgreSQL, was rejected

Motivation for this in existing DF logic is that DF does not provide default mechanism to attach args to specific supported arguments so they are provided consequently (first N args). Spice implements own mechanism for this (extra metadata) and can benefit from full Postgres dialect.

PR updates ordering logic to only validate that positional arguments fill consecutive slots from position 0. Named arguments can target any parameter position. This does NOT break existing functionality (no changes if arguments are specified in more conservative way) but also unblock Spice scenario:

// Before: for i in 0..args.len()        -- checked positions 0,1,2 for 3 args
// After:  for i in 0..positional_count  -- only checks positional arg positions

Example

vector_search(foo, 'bar', rank_weight => 100)
-- params: [tbl, query, column, limit, include_score, rank_weight]
-- 2 positional fill positions 0,1 ✓
-- Named 'rank_weight' fills position 5 ✓
-- Positions 2,3,4 skipped (optional) ✓

Important:

DataFusion only validates argument names and reorders to signature order. Functions are responsible for their own required/optional argument validation and must use metadata or other mechanisms to identify named arguments when positions are skipped.

PostgreSQL Reference

From PostgreSQL Docs - Calling Functions:

"In either notation, parameters that have default values given in the function
declaration need not be written in the call at all. But this is particularly
useful in named notation, since any combination of parameters can be omitted."