What this PR does / why we need it

This PR fixes an issue in the Redis online store where Feast drops sub-second
precision when comparing and storing event timestamps through the /push
endpoint.

Problem

Feast currently serializes event timestamps using only the seconds field of
the protobuf Timestamp (Timestamp.seconds), discarding Timestamp.nanos.
As a result:

• Two events occurring within the same second (e.g. 47.500Z and 47.700Z)
  are treated as having the same timestamp.
• The /push endpoint rejects later events as “older”, even when they occur
  milliseconds later.
• The online store cannot guarantee correct last-write-wins semantics for
  high-frequency sources (streaming pipelines, event collectors, etc.).

What this PR changes

✔ Store full protobuf Timestamp including nanos

online_write_batch now constructs timestamps using:

ts = Timestamp()
ts.FromDatetime(timestamp)

which preserves both seconds and nanos.

✔ Compare event freshness using nanosecond precision

Old behavior:

if new_seconds <= prev_seconds: skip

New behavior:

if new_total_nanos <= prev_total_nanos: skip

This ensures that sub-second updates are correctly accepted.

✔ Reconstruct full timestamp in online_read

Feast now returns proper millisecond-aware timestamps by combining
seconds + nanos / 1e9.

Result

Feast users can now push multiple updates that occur within the same second and
Redis will correctly:

• accept the newer event,
• store its full timestamp,
• and return precise timestamps via get_online_features.

This enables correct behavior for any use case emitting dense real-time data.

Which issue(s) this PR fixes

Fixes a long-standing limitation in the Redis online store where timestamp
precision was effectively second-level, causing incorrect deduplication and
event rejection when using the /push API.

(If maintainers want an issue link, I can open one in feast-dev.)

Misc

• Changes are fully backward compatible: existing keys without nanos continue to
  work as before.
• No API changes.
• Behavior is now aligned with Feast’s use of protobuf Timestamp in other
  online store implementations.