Add test for AdaptiveRetry free retries when shouldPayFailureCost is false

[adamw]

Reproduces issue #442: AdaptiveRetry incorrectly stops retrying on Left when shouldPayFailureCost returns false.

The test verifies that when shouldPayFailureCost is false, retry attempts should not consume from the token bucket. Currently, all 6 attempts (1 initial + 5 retries) are expected to succeed despite a token bucket with only 2 tokens, since no cost should be paid.

The fix implementation will follow.

Closes #442.

[adamw]

Found it. Here's the relevant excerpt:

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Test failure in AdaptiveRetryTest (Java 24 run):

AdaptiveRetryTest:
AdaptiveRetry
- should not pay failureCost if result E is going to be retried and shouldPayFailureCost returns false *** FAILED ***
  1 was not equal to 6 (AdaptiveRetryTest.scala:30)

The newly added failing test (should not pay failureCost if result E is going to be retried and shouldPayFailureCost returns false) is failing as expected — it was added to document the bug, not to pass. The assertion at line 30 gets 1 but expects 6, confirming the bug: AdaptiveRetry is stopping on a Left result even when shouldPayFailureCost returns false (i.e., the retry budget is being incorrectly debited on Left values that should not incur a failure cost).