Feature: ATTACH passthrough mode — forward SQL instead of decomposing into TVF calls

[asubbarao]

Use case

When using Airport to attach a remote DuckDB instance, the current behavior decomposes function calls into TVF descriptors with positional parameters. This works for custom server-defined functions, but breaks for DuckDB-native extension functions like read_csv, read_text, lsr, etc. that:

1. Have 40+ parameters with named parameter syntax (auto_detect:=true)
2. Depend on extensions loaded on the remote side (shellfs, hostfs, etc.)
3. Have dynamic output schemas that depend on the input

The server IS DuckDB — it can execute any SQL natively. airport_take_flight already sends raw SQL and works perfectly. But ATTACH + USE goes through TVF decomposition, which loses named parameters and requires the server to pre-register every function signature.

Proposal

Add an optional mode parameter to ATTACH:

ATTACH 'grpc://remote:8815' AS remote (TYPE airport, mode passthrough);
USE remote;
-- This SQL is forwarded as-is to the remote DuckDB, not decomposed into TVF calls
SELECT * FROM read_csv('cmd |', auto_detect:=true, header:=false);

In passthrough mode, when the attached catalog receives a query it can't resolve locally (or all queries), it forwards the entire SQL string as a CMD descriptor to the remote — the same path airport_take_flight uses.

The default TVF decomposition behavior stays unchanged for servers that define custom functions. Passthrough mode is for when the remote is a full DuckDB instance and you want ATTACH to be syntactic sugar for airport_take_flight.

Context

• PR fix: LookupSchema returns null for DEFAULT_SCHEMA, blocking table function resolution #47 (merged) fixed LookupSchema for DEFAULT_SCHEMA — USE remote and qualified function calls now work
• The remaining gap is that function calls still go through TVF decomposition instead of raw SQL forwarding
• This affects shellfs (read_csv('cmd |')), hostfs (lsr, read_text), and any extension function with named parameters

Does this make sense as an approach? I don't want to change the default TVF decomposition behavior — just add an optional mode for the remote-DuckDB use case.