Probed real bedrock (commercial us-east-1) with a 14713-token cached prefix over
streaming /v1/messages. Its raw message_delta already carries the cache breakdown
alongside the uncached input_tokens, so usage stays self-consistent, prompt_tokens
resolves to 14725 and the derived text_tokens to 12. Re-driving the same call with
the fix reverted logged positive spend (0.00166463, 0.00165363) and zero negative
rows, so _merge_message_start_cache_into_delta_usage is a no-op on this deployment.
The bug needs a deployment whose message_delta omits the breakdown (GovCloud /
LIT-2411), which a live commercial call cannot produce. Mark the path unsupported
for live e2e rather than a gap nobody can close, keep the mock unit test as its
guard, and write the evidence down so the probe is not repeated.