oauth_validator: Print captured stderr after call-count failure

Jacob Champion <jchampion@postgresql.org>

Commit: 1fb397f730b209edaff995f9cc411a5d8ed2d65f
Author: Jacob Champion <jchampion@postgresql.org>
Date: 2026-06-17T17:15:38Z
oauth_validator: Print captured stderr after call-count failure

If the call count test fails, you'll reasonably want to know what the
network trace looked like, but that information is currently swallowed.
Print it out instead.

Backpatch-through: 18

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