HIR planning: Also cast element type after CastInt2VectorToArray

[ggevay]

Fixes https://github.com/MaterializeInc/database-issues/issues/10087 and https://github.com/MaterializeInc/database-issues/issues/10088. The bug was that our
The Int2Vector to Array cast used to just blindly say that the cast succeeds by simply adding a call to CastInt2VectorToArray without regard to the Array's element type. The fix is to check the Array element type, and if it's not Int2, then additionally insert an array to array cast that attends to the element type (or fail the cast if the element types are not castable).

Btw. Junie (RustRover's AI agent) with Opus 4.6 oneshotted the fix in <5 minutes, which I think is super impressive. I only needed to adjust the tests and the code comment, plus manually tested Postgres compat.

[github-actions]

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