Migrate object handles from int to opaque EntityId values

[hatayama]

The Unity 6000.5 compatibility check currently preserves the existing int object-handle contract with a localized obsolete-diagnostic suppression in UnityObjectIdentifier.

Unity 6000.5 does not provide a supported EntityId-to-int conversion, so the long-term fix is to migrate the pause-point and related Go wire contracts to opaque EntityId-compatible values. This will require coordinated protocol and serialization changes.

[hatayama]

Status: deferred, kept open as the tracking issue.

Current state on main: UnityObjectIdentifier derives the int handle from EntityId.ToULong(GetEntityId()) under a localized obsolete-diagnostic suppression, and every supported Unity version (2022.3 through 6000.5) resolves the same object from that value. Nothing observable is broken today.

The migration to opaque EntityId values touches the pause-point and related Go wire contracts and therefore requires a protocol version bump (cli/common/clicontract/contract.json and CliConstants.REQUIRED_CLI_PROTOCOL_VERSION together), which we only do when the wire format actually becomes incompatible. Paying that cost now would force every user onto a coordinated CLI plus package upgrade for no user-visible gain.

Reversal condition, so this is not re-litigated from scratch: start the migration when either (a) a supported Unity version removes the int conversion or starts returning EntityId values that no longer round-trip through ToULong to a stable int within an Editor session, or (b) another wire-incompatible change is already scheduled and this can ride the same protocol bump.