Fix AdaptiveRetry stopping on Left when shouldPayFailureCost returns false

[adamw]

Fixes #442.

When AdaptiveRetry encounters a Left error and shouldPayFailureCost returns false, retrying should continue. Currently, the retry stops in this scenario.

This commit adds a failing test that demonstrates the expected behavior: retries should continue for the configured schedule even when the cost is not paid (shouldPayFailureCost returns false).

Closes #442.

[adamw]

The failure is clear. Here's the summary:

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CI failure: ImmediateRetryTest — one test, both Java 21 and 24

- should not pay exceptionCost if Left error is going to be retried
  and shouldPayPenaltyCost returns false *** FAILED ***
  1 was not equal to 6 (ImmediateRetryTest.scala:228)

The test asserts that when shouldPayPenaltyCost returns false and the result is a Left that will be retried, the penalty cost counter stays at 6 (untouched across retries). Instead it is 1, meaning the implementation is charging the penalty cost on the first attempt and stopping, rather than skipping the cost and continuing to retry. This is the intentional failing test added to document the bug this PR is meant to fix.