kad: Fix local provider eviction and removal panics

[DenzelPenzel]

Fixes #665.

• Keep local providers separate from remote records so eviction or expiry cannot remove an active local registration or cause a panic on removal.
• Enforce separate local/remote provider-key limits and report QueryFailed without publishing rejected registrations.
• Add regression coverage for eviction, expiry, refresh at capacity, and registration rejection.

[gab8i]

With this change max_provider_keys bounds remote keys and local registrations independently, so a node can hold up to 2 × max_provider_keys in total. This should be documented in the docs of MemoryStoreConfig::max_provider_keys, with_max_provider_keys, and in the changelog. Alternatively, would a dedicated limit for local keys be clearer than reusing the same value?

[DenzelPenzel]

With this change max_provider_keys bounds remote keys and local registrations independently, so a node can hold up to 2 × max_provider_keys in total. This should be documented in the docs of MemoryStoreConfig::max_provider_keys, with_max_provider_keys, and in the changelog. Alternatively, would a dedicated limit for local keys be clearer than reusing the same value?

Thanks for the review! Went with documenting the current behaviour

[AndreiEres]

Looks good

[gab8i]

LGTM!