Net 682 split assignment

[toschoosqd]

Description

Linear: NET-682

Adds feature-gated support for split assignment discovery in sqd-assignments.

This introduces the mvcc-chunks feature flag and extends NetworkState with optional worker_assignment and portal_assignment fields when the feature is enabled. The existing assignment field remains unchanged and always present for backward compatibility.

Also formalizes the deprecated NetworkAssignment::url field:

• marks it with #[deprecated];
• defaults it during deserialization;
• omits it during serialization when absent.

Includes JSON compatibility tests covering:

• legacy NetworkState deserialization;
• split assignment deserialization under mvcc-chunks;
• omitted split assignment fields remaining omitted after JSON round trip;
• deprecated url defaulting to None and being skipped when absent.

Small unrelated formatting/readability cleanup:

• replaces implicit peer_id.into() dialing with explicit DialOpts::peer_id(peer_id).build().

Validation:

• cargo fmt --check
• cargo test -p sqd-assignments
• cargo test -p sqd-assignments --features mvcc-chunks
• cargo check -p sqd-network-transport

Note

Reminder before the scheduler PR: update network-scheduler’s sqd-assignments dependency from local path to the relevant sqd-network commit.

[define-null]

LGTM