bugfix(livestore): skip lookback replay when partition is Inactive

[zhxiaogg]

What this PR does:

• If a Livestore partition is marked as Inactive, due to /prepare-partition-downscale, then the Livestore instance should not replay from the beginning of the partition for any intermediate restarts.

Why:
This is to avoid partition lag due to concurrent scaling down and rolling restarts:

1. when downscaling starts, the /prepare-partition-downscale would set target partitions to be inactive
2. the corresponding livestore pod is turning into readonly until all the WAL blocks are expired
3. if there happen to be any rolling restarts and all WAL blocks are expired, the livestore might replay from the beginning of the partition. This is unexpected.
4. subsequent downscaling procedures still could succeed, but there could be partition lag due to the unexpected lookback replaying. This PR is to avoid the unexpected replaying for any intermediate rolling restarts.

Which issue(s) this PR fixes:
Fixes N/A

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[Copilot]

Pull request overview

This PR adjusts live-store Kafka startup behavior so that when a live-store’s ingest partition is already marked Inactive (typical during downscaling/draining), the process does not force a lookback replay solely because local WAL-derived state is empty after restart. This prevents expensive/unnecessary replay during intermediate restarts while a scale-down is in progress.

Changes:

• Refactors the “force lookback replay when no local instances exist” decision into shouldForceFromLookback(ctx).
• Skips forced lookback replay when the partition ring reports the partition state as PartitionInactive.
• Adds unit tests covering the three key branches (instances exist, no instances + inactive, no instances + non-inactive) and updates the changelog.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
modules/livestore/live_store.go	Introduces shouldForceFromLookback() and uses partition state to skip lookback replay when Inactive.
modules/livestore/live_store_test.go	Adds tests validating the new decision logic for lookback replay.
CHANGELOG.md	Adds a [BUGFIX] entry documenting the behavior change.

[mattdurham]

lgtm