[net-685] last applied assignment added

[toschoosqd]

Summary

Implements the ping-collector portion of [NET-685] behind the mvcc-chunks feature flag.

• Stores last_applied_assignment_id from worker heartbeats.
• Adds Nullable(Int64) to fresh worker_pings_v2 tables.
• Migrates existing tables idempotently with explicit column placement.
• Preserves missing values as NULL.
• Rejects non-positive assignment IDs.
• Documents append-only, max-per-worker consumption semantics.

Dependencies

• NET-686: defines the strictly monotonic assignment ID contract.
• sqd-network PR: adds optional last_applied_assignment_id to Heartbeat.
• The sqd-network commit reference must be updated after that protocol PR is committed.
• Worker-side NET-685 changes must populate the heartbeat field.
• NET-681 / NET-684 consume the ClickHouse signal downstream.

Feature Flag

Enabled with:

mvcc-chunks

Legacy builds retain the existing ClickHouse row shape and behavior.

Validation

• Legacy collector tests: 4 passed, 1 ClickHouse test ignored.
• Feature-enabled tests: 7 passed, 1 ClickHouse test ignored.
• Legacy and feature-enabled pings-collector checks passed.
• sqd-messages tests passed.
• git diff --check passed.

[define-null]

Generally LGTM, left a bunch of suggestions.