Close the last tenant-isolation adversarial probe (mid-provisioning tenants)

Phase 4 task 8's verification plan named four adversarial probes;
three were closed earlier this phase, the fourth (an evaluator tick,
or any other caller, hitting a tenant that exists but hasn't reached
the active+credentialed gate yet -- must be refused, not served) was
still an explicitly-skipped stub in
api/queryapi/tenant_isolation_gap_test.go.

Investigating it found the two storage engines needed genuinely
different treatment:

- ClickHouse (enterprise/internal/chrunner) already had this property
  structurally, for free: Registry is built once at startup from
  rbacstore.ListProvisionedDataSources, which already filters to
  active+credentialed tenants only, so a mid-provisioning tenant is
  simply absent from the connection map. New test
  TestRegistryRefusesMidProvisioningTenant proves this without Docker
  -- an empty DataSource list never dials ClickHouse, so this genuinely
  runs in this environment, unlike every other test in that file.

- Tantivy (search/src/registry.rs's IndexRegistry) was a real, different
  gap, not just an unverified assumption: it opens-or-creates an index
  for any syntactically-valid tenant_id on first request, because it's
  a separate process with no Postgres access and structurally can't
  know which tenants are actually provisioned. A query against a
  mid-provisioning tenant would have silently succeeded with zero
  results from a freshly-created empty index -- "ambient success"
  indistinguishable from "no matching logs," exactly the failure mode
  this item was worried about.

Fixed the Tantivy gap with a new enterprise/internal/searchclient.
TenantChecker interface (backed by a new rbacstore.TenantIsActive,
implemented structurally, no new import edge needed), consulted before
every gRPC call: Client.Search now refuses a non-active tenant before
it ever reaches `search`. Dial's signature gained a required
TenantChecker parameter; enterprise-api's main.go passes its existing
rbacstore.Store (already satisfies the interface). Verified Docker-free
via searchclient's existing real-in-process-gRPC-server test harness
(TestSearchRefusesMidProvisioningTenant, plus
TestSearchPropagatesTenantCheckerError for the fail-closed-on-error
case) -- both genuinely run in this environment, same bar as the rest
of the Tantivy isolation work.

rbacstore.TenantIsActive itself has two new skip-gated live-Postgres
tests (TestTenantIsActive, TestTenantIsActiveNonexistentTenant) --
disclosed as not run against a live database here, same gap as the
rest of this phase's Postgres-backed pieces.

api/queryapi/tenant_isolation_gap_test.go rewritten from a checklist
with one skipped stub to a full accounting of all four now-closed
probes. Docs updated in lockstep: CLAUDE.md, threat-model.md,
phase-4-isolation-design.md (implementation note added after its
original sign-off), phase-4-runbook.md (§9), enterprise/README.md.

enterprise/internal/chrunner/chrunner_test.go

@@ -183,3 +183,38 @@ func TestRegistryTenantCannotReadOtherTenantEvenViaRawSQL(t *testing.T) {
 		t.Fatal("tenant A's request was able to read tenant B's database by fully-qualified name -- isolation is broken")
 	}
 }
+
+// TestRegistryRefusesMidProvisioningTenant is Phase 4 task 8's item 4
+// adversarial probe (see /docs/phase-4-isolation-design.md's
+// verification plan and api/queryapi/tenant_isolation_gap_test.go):
+// a tenant row that exists in rbacstore but hasn't reached the
+// active+credentialed gate yet must be refused, not served via some
+// ambient connection. Unlike every other test in this file, this one
+// needs no live ClickHouse at all -- New never dials out for an empty
+// DataSource list, so an empty Registry (as if every tenant in
+// `tenants` were still mid-provisioning) is exactly what
+// enterprise-api's main.go would build from
+// rbacstore.ListProvisionedDataSources before any tenant clears that
+// filter. "Mid-provisioning" and "entirely unknown" collapse to the
+// identical code path here by construction: Registry has no concept of
+// "a tenant row exists," only of "a runner is in my map" -- the real
+// gate is ListProvisionedDataSources's SQL WHERE clause, already
+// covered by rbacstore_test.go's
+// TestListProvisionedDataSourcesExcludesUnprovisionedAndInactive. This
+// test is the Docker-free proof that RunSQL's refusal actually holds on
+// the empty-map end of that gate, complementing
+// TestRegistryRefusesUnknownTenant's live-ClickHouse proof on the
+// populated end.
+func TestRegistryRefusesMidProvisioningTenant(t *testing.T) {
+	ctx := context.Background()
+	reg, err := New(ctx, "unused:9000", nil)
+	if err != nil {
+		t.Fatalf("New: %v", err)
+	}
+	defer reg.Close()
+
+	reqCtx := authz.WithIdentity(ctx, authz.Identity{TenantID: "mid-provisioning-tenant", Role: authz.RoleViewer})
+	if _, err := reg.RunSQL(reqCtx, "SELECT 1"); err == nil {
+		t.Fatal("expected RunSQL to refuse a tenant that hasn't reached the active+credentialed gate, not silently serve it")
+	}
+}