Remove timestamp fallbacks for non-batched publishing

[mjohanse-emr]

What does this Pull Request accomplish?

This PR removes the timestamp fallbacks from non-batched publish measurement.

Before this change, if timestamp was not provided, we would fall back to one of these two values:

1. Waveform t0 if we are publishing a waveform and t0 is set.
2. The current time if we are publishing a waveform without t0 or publishing any other type.

The desire is to conceptually move this logic into the Datastore Server. The implementation may not look exactly the same, but the goal on the client side is just to pass through the provided timestamp without altering it.

Why should this Pull Request be merged?

AB#3926275

We want to remove the timestamp fallbacks until we decide how to handle it on the server side.

What testing has been done?

I updated two unit tests to check for a default PrecisionTimestamp if no timestamp is provided.
All unit tests and acceptance tests pass.

[Copilot]

Pull request overview

This PR removes client-side timestamp fallback behavior for non-batched publish_measurement calls so that the client passes through a caller-provided timestamp (or omits it when not provided), delegating any defaulting logic to the Datastore Server.

Changes:

• Removed waveform-t0 and “now” timestamp fallback logic from gRPC conversion, replacing it with a simple “convert if provided” helper.
• Updated DataStoreClient.publish_measurement to set the request timestamp directly from the optional caller-provided timestamp (no fallback).
• Updated unit tests to expect a default/empty PrecisionTimestamp when no timestamp is provided.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
tests/unit/data/test_publish_measurement.py	Updates unit tests to align expectations with “no client-side timestamp fallback” behavior.
src/ni/datastore/data/_grpc_conversion.py	Removes fallback timestamp selection logic and adds a simple optional timestamp conversion helper.
src/ni/datastore/data/_data_store_client.py	Updates non-batched publish path to stop applying fallback timestamps client-side.

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[dixonjoel]

We discussed that we'll wait until the corresponding LabVIEW changes are approved to submit this PR, so we ensure that if that one doesn't make it, we're not making consistency worse by submitting this PR.