Closes apache#1002. v3 spec requires `"current-snapshot-id": null` for empty
tables, not the field omitted entirely (Java apache/iceberg#12335).
omitempty on the *int64 field was dropping the key when nil; metadataV3
now has a MarshalJSON that re-injects the key with explicit null after
the snapshots field — Java's declaration order — while preserving v1/v2
omitempty behaviour, since v1/v2 fixtures rely on the -1 sentinel.

The injection walks the marshaled JSON via json.Decoder so the
surrounding field order is preserved (a map round-trip would resort
alphabetically and break byte-level diffs against Java-produced
metadata.json).

Closes apache#1002. v3 spec requires `"current-snapshot-id": null` for empty
tables, not the field omitted entirely (Java apache/iceberg#12335).
omitempty on the *int64 field was dropping the key when nil; metadataV3
now has a MarshalJSON that re-injects it as explicit null at its
canonical declaration-order position — just before the first key that
follows it in commonMetadata — so empty-table output stays
byte-compatible with Java's metadata.json shape. v1/v2 keep omitempty
behaviour, since v1/v2 fixtures rely on the -1 sentinel.

The injection walks the marshaled JSON via json.Decoder so the
surrounding field order is preserved (a map round-trip would resort
alphabetically and break byte-level diffs against Java-produced
metadata.json).

Same MarshalJSON adds a symmetric guard for last-partition-id, which is
required for v2+ per spec but had the same omitempty tag. The parser's
preValidate/validate path already enforces non-nil on read; this is the
write-side equivalent so direct struct construction that skips the
builder can't silently emit a non-conformant file Java/PyIceberg would
reject. In practice the builder always sets the field, so the guard
only fires on malformed in-memory state.

Closes apache#1002. v3 spec requires `"current-snapshot-id": null` for empty
tables, not the field omitted entirely (Java apache/iceberg#12335).
omitempty on the shared commonMetadata field was dropping the key when
nil; metadataV3 now has a MarshalJSON that wraps the type alias inside
an outer struct redeclaring CurrentSnapshotID without omitempty, so the
field-name collision resolves to the shallower (always-emitted)
declaration. v1/v2 keep the omitempty path since their fixtures rely on
the -1 sentinel.

Same MarshalJSON adds a symmetric guard for last-partition-id, which is
required for v2+ per spec but had the same omitempty tag. The parser's
preValidate/validate path already enforces non-nil on read; this is the
write-side equivalent so direct struct construction that skips the
builder can't silently emit a non-conformant file Java/PyIceberg would
reject. In practice the builder always sets the field, so the guard
only fires on malformed in-memory state.

Closes apache#1002. v3 spec requires `"current-snapshot-id": null` for empty
tables, not the field omitted entirely (Java apache/iceberg#12335).
omitempty on the shared commonMetadata field was dropping the key when
nil; metadataV3 now has a MarshalJSON that wraps the type alias inside
an outer struct redeclaring CurrentSnapshotID without omitempty, so the
field-name collision resolves to the shallower (always-emitted)
declaration. v1/v2 keep the omitempty path since their fixtures rely on
the -1 sentinel.

Same MarshalJSON adds a symmetric guard for last-partition-id, which is
required for v2+ per spec but had the same omitempty tag. The parser's
preValidate/validate path already enforces non-nil on read; this is the
write-side equivalent so direct struct construction that skips the
builder can't silently emit a non-conformant file Java/PyIceberg would
reject. In practice the builder always sets the field, so the guard
only fires on malformed in-memory state.

Closes #1002. v3 spec requires `"current-snapshot-id": null` for empty
tables, not the field omitted entirely (Java apache/iceberg#12335).
omitempty on the *int64 field was dropping the key when nil; metadataV3
now has a MarshalJSON that re-injects the key with explicit null after
the snapshots field — Java's declaration order — while preserving v1/v2
omitempty behaviour, since v1/v2 fixtures rely on the -1 sentinel.

The injection walks the marshaled JSON via json.Decoder so the
surrounding field order is preserved (a map round-trip would resort
alphabetically and break byte-level diffs against Java-produced
metadata.json).