fix: handle EXISTS/NOT EXISTS subqueries in federation analyzer

[sgrebnov]

🗣 Description

Cross-provider queries with EXISTS/NOT EXISTS subquery filters fail because the federation analyzer doesn't inspect tables inside Expr::Exists. As a result the analyzer only sees the outer query's tables, concludes single provider, and federates the entire query — including the subquery's tables — to one DBMS, where the subquery's table doesn't exist (e.g., no such table: lineitem)

🔨 Related Issues

Fixes spiceai/spiceai#10167

🤔 Concerns

[Copilot]

Pull request overview

Fixes federation analysis for cross-provider queries that use EXISTS/NOT EXISTS predicate subqueries by ensuring the analyzer inspects tables referenced inside Expr::Exists and federates subqueries correctly.

Changes:

• Extend provider scanning to traverse Expr::Exists subquery plans when determining whether a subtree is single-provider or cross-provider.
• Extend expression rewriting to recursively analyze/federate EXISTS subqueries, including a compatibility workaround for optimizer rules by wrapping federated subqueries in a no-op Projection when needed.

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