Scope in-memory uniqueness maps by Kind in ConfigStore

Description

The in-memory uniqueness maps cs.handle and cs.nameVersion in ConfigStore (gateway/gateway-controller/pkg/storage/memory.go) enforce global uniqueness across all configuration kinds, but GetByKindAndHandle and GetByKindNameAndVersion scope their lookups by Kind. This means two configurations of different kinds (e.g., RestApi and LlmProvider) with the same handle or name:version would incorrectly conflict on Add, even though they are logically distinct.

Expected behavior: Uniqueness for handle and name:version should be enforced per-Kind. A RestApi with handle "my-api" and an LlmProvider with handle "my-api" should coexist.

Actual behavior: The second Add call fails with ErrConflict because the maps are flat (handle → UUID, name:version → UUID) without Kind scoping.

Fix: Update cs.handle and cs.nameVersion to use composite keys that include cfg.Kind (e.g., kind:handle and kind:displayName:version). Update Add, Update, Delete, and the Kind-scoped retrieval functions accordingly. The cross-kind GetByHandle and GetByNameVersion should fall back to scanning cs.configs.

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