An extracted entry whose tar header mtime is 0 was stored with
mod_time = UNIX_EPOCH, which xl.meta encodes as 0 nanos (= no
mod_time). On read-back the version failed valid() and parsing fell
into the legacy rmp_serde fallback, so every read of the object
returned 500 (invalid type: integer 0, expected an OffsetDateTime).
Treat mtime 0 as unset (tar convention) and fall back to the upload
time. Also fix two test-side issues uncovered behind the 500: the pax
fixture must use a ustar header for its XHeader entry, and the SSE-S3
extract tests must provision RUSTFS_SSE_S3_MASTER_KEY. Re-admit the 19
quarantined tests to the e2e-full merge gate.
Fixes#4842
