feat(api): default KVCacheBackend leader.multiTenancy to true

[thxCode]

What type of PR is this?

/kind enhancement
/kind api-change
/area worker

What this PR does / why we need it:

Defaults KVCacheBackend's leader.multiTenancy to true (*bool with +k8s:validation:default=true; every reader goes through MultiTenancyEnabled(), which reads an unset field as the schema default, so an object that never passed the API server answers the same as one that did).

The tenant ledger is what makes the rest of this API mean what it says: without it a KVCachePoolBinding's ceiling is recorded but not enforced, and a master serves one reuse domain only, so a second Binding on it is refused. Mooncake has taken the -enable_multi_tenants switch since 0.3.12 — verified against the upstream tags: absent from v0.3.10.post2 and v0.3.11, present from v0.3.12 on — and the default store image is on 0.3.13.post1. KVCacheBackend has not shipped in any release tag, so the default changes before anyone upgrades into it.

Omitting the key and writing multiTenancy: false are now different things: only the explicit false renders no switch. A store image older than Mooncake 0.3.12 does not recognize the switch and its master exits at startup, so a backend on such an image — including the 0.3.10.post2 variants this project publishes — sets false explicitly.

Docs: backend.md records the 0.3.12 floor beside the build variants that need the opt-out, and the walkthrough's Step-1 backend now reads as the multi-tenant shape it renders.

Which issue(s) this PR links to:

None.

Special notes for your reviewer:

Upgrade note: an existing KVCacheBackend that never set the field reads as true under this build, so its leader is re-rendered with -enable_multi_tenants=true on the first reconcile after the upgrade — the leader restarts and the cached contents do not survive. A backend pinned to a store image older than Mooncake 0.3.12 must set leader.multiTenancy: false before the operator is upgraded, or its master exits at startup on the unknown flag.

Does this PR introduce a user-facing change?

action required: `KVCacheBackend`'s `leader.multiTenancy` now defaults to `true`. A backend pinned to a Mooncake store image older than 0.3.12 must set `leader.multiTenancy: false` explicitly — the default renders a switch such a master does not recognize, and it exits at startup. Any other backend that never set the field gains the per-tenant quota ledger on the first reconcile after the upgrade, which restarts its leader and does not preserve cached contents.

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