with_session_state silently falls back when Session isn't SessionState

[ethan-tyler]

What happened?

Builders like OptimizeBuilder, MergeBuilder, UpdateBuilder accept Arc via with_session_state(...), but internally require a concrete SessionState. When the downcast fails, they silently fall back to an internal default:

session.as_any().downcast_ref::<SessionState>()
    .cloned()
    .unwrap_or_else(|| create_internal_session_state(...))

Relevant paths:

• crates/core/src/operations/optimize.rs
• crates/core/src/operations/merge/mod.rs
• crates/core/src/operations/update.rs

The operation "works", but caller-provided config/runtime/registrations get silently ignored.

Expected behavior

Either accept SessionState directly, error when downcast fails, or at minimum log/document that fallback happened.

Operating System

All (API-level behavior)

Binding

Rust

Bindings Version

Observed on main / DF52 upgrade work (DataFusion 52).

Steps to reproduce

1. Create a session with custom config (runtime env, target partitions, UDF registrations, etc.)
2. Pass it to OptimizeBuilder::with_session_state(...) as Arc where concrete type isn't SessionState
3. Operation runs with internal defaults instead of caller's session - no warning

Relevant logs

N/A (the problem is the absence of any warning/error when fallback happens)

[ethan-tyler]

take