fix: size cache key buffer per rule in cached enforcers

[zjncs]

Description

CachedEnforcer.RemovePolicies and SyncedCachedEnforcer.checkManyAndRemoveCache (used by both AddPolicies and RemovePolicies) size the cache-key buffer from the first rule of the batch and reuse it for every rule:

irule := make([]interface{}, len(rules[0]))
for _, rule := range rules {
    for i, param := range rule {
        irule[i] = param
    }
    key, _ := e.getKey(irule...)
    ...
}

When the batch contains rules of different lengths (the [][]string API does not require uniform length, and the model accepts variable-length rules — e.g. an ABAC condition field makes rules longer than the plain RBAC ones):

• a later rule longer than the first panics with index out of range;
• a later rule shorter than the first keeps stale elements from the previous rule at the tail, so the computed cache key is wrong (e.g. alice$$data1$$read$$extra instead of alice$$data1$$read$$) and the real cached decision of a removed rule is never deleted.

The second case is an authorization correctness bug, not just hygiene: after RemovePolicies removes a rule from the model, Enforce keeps returning the stale cached true:

e, _ := NewCachedEnforcer("examples/rbac_model.conf", "examples/rbac_policy.csv")
_, _ = e.AddPolicies([][]string{{"bob", "data2", "write", "extra"}})
e.Enforce("alice", "data1", "read") // true, cached
_, _ = e.RemovePolicies([][]string{
    {"bob", "data2", "write", "extra"},
    {"alice", "data1", "read"},
})
e.Enforce("alice", "data1", "read") // still true — rule was removed but the cached decision survived

Fix: allocate the key buffer per rule (make([]interface{}, len(rule)) inside the loop) in both enforcers.

Testing

Added 4 tests (TestRemovePoliciesCacheRaggedRules{StaleEntry,LongerRule} in enforcer_cached_test.go and the TestSync... twins in enforcer_cached_synced_test.go).

Before the fix (master 34297a1):

• StaleEntry tests FAIL: alice, data1, read: true, supposed to be false — stale cached decision
• LongerRule tests FAIL: panic: runtime error: index out of range [3] with length 3 in checkManyAndRemoveCache/RemovePolicies

After the fix: all 4 pass. All existing cache tests (TestCache, TestSyncCache, cached g-function tests) still pass, and the full go test ./... suite passes (8 packages). go vet clean.

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