Include current error when retry attempts are exhausted

[himanalot]

Fixes #186.

When an inner call failed, the retry loop checked retries_exhausted before pushing the current error into errors. With retry = 0, this returned AllAttemptsErrored([]) and dropped the underlying failure.

This records the current error before returning the exhausted-attempts error, for both request failures and client recreation failures. It also adds regression coverage for retry = 0 and accumulated retry errors.

Testing:

• Not run locally (cargo is not installed in this environment).