[C++] Incorrect vector capacity check in TDigest::TDigestImpl::Validate() #53777

Describe the bug, including details regarding any error messages, version, and platform.

The check at https://github.com/apache/arrow/blob/main/cpp/src/arrow/util/tdigest.cc#L172:

    // check if buffer expanded
    if (tdigests_[0].capacity() > delta_ || tdigests_[1].capacity() > delta_) {
      return Status::Invalid("oversized tdigest buffer");
    }

seems meant to verify that inserting into the buffers up to the previously reserved capacity didn't cause a reallocation to a larger capacity. However, the check is assuming that std::vector::reserve uses an exact capacity reservation, which is not guaranteed (https://en.cppreference.com/cpp/container/vector/reserve). We should update or remove this check, which relies on implementation details of std::vector that are subject to change.

Motivation: I'm working on using size-returning-new in std::vector::reserve, which causes this check to fail.

(See sample fix in #52276.)

Component(s)

C++