fix(redis-schema): copy policy schemas instead of mutating shared tables

[janiussyafiq]

Description

apisix/utils/redis-schema.lua builds limit-conn's key_ttl schema variants by mutating the shared policy_to_additional_properties tables in place:

local limit_conn_redis_cluster_schema = policy_to_additional_properties["redis-cluster"]
limit_conn_redis_cluster_schema.properties.key_ttl = {
    type = "integer", default = 3600,
}

Since this runs at module load, key_ttl (and its default of 3600) leaks into the base redis / redis-cluster tables that every other consumer of this module reuses. Observable effect today: limit-count and limit-req confs silently gain a key_ttl = 3600 field during schema validation (defaults are applied to the conf), even though neither plugin understands the property. limit-count's core.table.deepcopy(redis_schema.schema) doesn't help — it copies the already-mutated tables.

This PR builds the limit-conn variants from a shallow copy of the shared base, so the extra property stays local to limit-conn:

• the shared redis / redis-cluster tables no longer carry key_ttl
• limit-conn behavior is unchanged (key_ttl still accepted, default 3600 still applied)
• limit-count / limit-req confs no longer get a foreign key_ttl injected; a stray explicit key_ttl on those plugins is still tolerated (their schemas do not reject additional properties) — it is just no longer injected

New regression test t/utils/redis-schema.t (the existing home for apisix/utils/* tests, alongside batch-processor.t / rfc5424.t): one block validates limit-count/limit-req confs across redis and redis-cluster policies — with limit-conn loaded first so any shared-table leak would be visible — and asserts no key_ttl is injected; a second block pins limit-conn's own behavior (default 3600 applied, explicit value honored). On unfixed master the first block fails with limit-count redis got key_ttl: 3600 (and the three other combinations); with the fix all cases pass.

This also unblocks new consumers of redis-schema.

Which issue(s) this PR fixes:

None (pre-existing latent bug found while building on redis-schema).

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change (no doc change needed: internal schema utility; user-facing behavior of limit-conn unchanged)
• I have verified that this change is backward compatible