only apply Range to a 206 response in write_content_with_provider

[metsw24-max]

Unvalidated Range reaches the content provider on non-2xx responses

write_content_with_provider slices the representation whenever req.ranges is non-empty, but apply_ranges only slices for a 206 and detail::range_error only validates the ranges for a 2xx status, so a handler answering 403 through a content provider hands the client's raw offset straight down (ASAN: heap-buffer-overflow, READ of 201 bytes past a 7-byte buffer, which then goes out on the socket; the bounds asserts in get_range_offset_and_length are compiled out under NDEBUG). Gating on 206 to match apply_ranges also settles the 2xx-but-not-206 case, where the body written was shorter than the Content-Length already sent, and the added test covers both.