Main is broken

[xudong963]

Example failure
https://github.com/apache/datafusion/actions/runs/14740728702/job/41378119017

     Running bin/sqllogictests.rs (target/ci/deps/sqllogictests-f0f60dad4353b70d)
External error: query failed: DataFusion error: Optimizer rule 'optimize_projections' failed
caused by
Check optimizer-specific invariants after optimizer rule: optimize_projections
caused by

[gstvg]

After #15149, flatten stopped working for List(FixedSizeList) because it expected the inner fixed size list to be casted to list, which wasn't happening anymore.

Looking back, #15160 correctly stopped using ArrayFunctionSignature::RecursiveArray, which I expected would make only the top level list be coerced, and it would break, but it didn't. It appears that recursive lists continued to be coerced

And then #15149 refactored that code path with the correct behavior, exposing the issue in flatten

I opened #15898 with a potential fix. Other options include:

1. Reverting to ArrayFunctionSignature::RecursiveArray again, but for deeply recursive fixed size lists, it would unnecessarily coerce some of them(which includes building offsets buffers).
2. Creating a new ArrayFunctionSignature variant for this case
3. Manually handling fixed size list in flatten