Duplicated parsing in tdf_reader.py #190

Some duplicated parsing was introduced by #188 .

is_tdf reimplements the same payload.get("url") extraction that payload_url_from_manifest_json already provides — it could just call that helper directly.

Note: Manifest.from_json is not a duplicate of that: it does full structural validation (raises on missing encryptionInformation, policy, segments, etc.), which is a different job than the lightweight "just peek at payload.url" probe.