sql: reject equal bounds in width_bucket

[virajchogle]

Fixes #171263

PostgreSQL rejects width_bucket(operand, b1, b2, count) when the lower and upper bounds are equal with the message "lower bound cannot equal upper bound" (pgcode invalid_argument_for_width_bucket_function, 2201G). CockroachDB silently returned garbage: the helper computes width = (b2 - b1) / count = 0, then int(math.Floor(diff/0) + 1) casts +Inf, -Inf, or NaN to int, producing INT64_MAX, INT64_MIN, or 0 depending on operand position.

This adds the guard to both the numeric and int overloads, using the same pgcode that the sibling NaN and finite-bound checks already raise. In the numeric overload, the guard is placed after the existing NaN/finite-bound guards and before the +/-Infinity operand shortcuts, matching PostgreSQL's check precedence.

This is a sibling of #171234 (count <= 0), already addressed by #171259, but independent and self-contained.

Epic: none

Release note (bug fix): width_bucket(operand, b1, b2, count) now returns "lower bound cannot equal upper bound" when the two bounds are equal, matching PostgreSQL.

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This change is 

[virajchogle]

Thanks for the review @rafiss! Could you please /trunk merge when you have a moment?

[rafiss]

Could you please /trunk merge when you have a moment?

Thanks for your contribution. You need to rebase this change and resolve merge conflicts first.

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[virajchogle]

@rafiss rebased onto master. Resolved conflicts. Ready to retry the merge.

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