feat(shard distributor): use smoothed load for initial ephemeral shard placement

[AndreasHolt]

What changed?

• switch assignEphemeralShard to call a new pickLeastLoadedExecutor helper that sums smoothed shard load per executor (falling back to shard count) and log the selected target
• cover the helper and the new load-based behavior in handler_test.go to make sure we pick the least-loaded executor and handle empty states
• add AggregateLoad and AssignedCount tags so handler logs can show load totals when placing ephemeral shards

Why?

• Initial placement previously picked the executor with the fewest assigned shards. Using the smoothed per-shard load lets us balance based on actual work.
• Logging aggregated load and assignment count for every placement call gives us observability when verifying decisions.

How did you test it?

• Added unit tests TestPickLeastLoadedExecutor and ShardNotFound_Ephemeral_LoadBased in handler_test.go to verify logic.
• Verified that when loads are equal (or zero), the logic correctly falls back to the fewest assigned shards.

Potential risks
If shard stats are missing or stale, the aggregated load will calculate as zero. In this case, the logic degrades to the previous behavior (selecting based on shard count), minimizing the risk of bad placement.

Release notes
Ephemeral shard placement now favors the executor with the lowest smoothed load (with shard-count tie breaker) and logs the inputs for each decision.

Documentation Changes