feat: Add checked_element_at for Spark ANSI mode array/map access

[n0r0shi]

Summary

Add checked_element_at that throws on array out-of-bounds access and missing
map keys, instead of returning null. This matches Spark's ANSI mode behavior
where ElementAt with failOnError=true raises exceptions.

Reuses the existing SubscriptImpl template with allowOutOfBound=false and
throwOnMissingMapKey=true. Both error paths support TRY() wrapping.

Test plan

• Array: normal access, out-of-bounds throws, TRY() returns null
• Map: normal access, missing key throws, TRY() returns null

Closes #16358

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[n0r0shi]

Added a doc. Ready for a review.