fix(dstack-types): read the whole sha256sum filename, not its first token

[kvinwang]

Problem

sha256sum_entry_hash took the filename as the second whitespace token and dropped the rest of the line. For <h1> measurement.tdx.cbor <h2> measurement.tdx.cbor it returned h1, while sha256sum -c (filename runs to end of line) matches neither entry, and the hidden second entry escaped the duplicate check. sha256sum.txt is what os_image_hash commits to, so this parser and sha256sum -c / a human reviewer should agree on what it says. Found by property tests (#1249).

Fix

Split on the first whitespace run and take the rest of the line as the filename. Well-formed <hash> <name> manifests parse identically.

Verification

Regression test added; cargo test -p dstack-types passes; fmt and clippy clean.

[kvinwang]

Superseded by 8a71300 on next, which parses sha256sum.txt with one strict grammar.