fix: check HTTP status code and fix concurrent fetch race

[beck-8]

https://filecoinproject.slack.com/archives/CPFTWMY7N/p1774514730698809

[rjan90]

Hmmmm... It looks like this PR exposed an existing race in concurrent param fetches:

TestGetParamsParallel runs multiple GetParams calls against the same cache dir. One caller finishes downloading a file, then a slower waiter retries the same file with Range: bytes=<full-file-size>-, and the gateway returns 416 Requested Range Not Satisfiable. Before this change that response was getting misclassified later as a checksum problem; now we correctly return the real HTTP error, so the existing race shows up in CI.

The smallest fix is probably to treat 416 as “the local file may already be complete” and re-validate the existing file before failing.