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.

CLAUDE.md

@@ -173,7 +173,23 @@ built too
 **genuinely verified**, like the OIDC login flow: Tantivy is an embedded
 library, not a networked service, so the isolation probe (three tenants,
 same search term, scoped search returns only that tenant's document)
-actually ran in this environment, no Docker needed. The deployment-
+actually ran in this environment, no Docker needed. That same
+Docker-free advantage is what caught a real bug while closing the last
+of Phase 4 task 8's four adversarial probes (a mid-provisioning tenant
+must be refused, not served): `search/src/registry.rs`'s `IndexRegistry`
+opened-or-created an index for any syntactically-valid `tenant_id`,
+meaning a query against a tenant that exists in `rbacstore` but isn't
+active yet would have silently returned zero results from a
+freshly-created empty index instead of being refused --
+`chrunner`'s ClickHouse routing had the equivalent guarantee for free
+(a mid-provisioning tenant simply isn't in its startup-built connection
+map) but Tantivy, a separate process with no Postgres access, had no
+way to know. Fixed with a new `enterprise/internal/searchclient.
+TenantChecker` (backed by `rbacstore.TenantIsActive`); both halves of
+the fix verified Docker-free (`chrunner_test.go`'s and
+`searchclient_test.go`'s `TestSearchRefusesMidProvisioningTenant`-shaped
+tests) — see `api/queryapi/tenant_isolation_gap_test.go` for the full
+accounting of all four probes, now all closed. The deployment-
 topology gap that briefly was the largest one is now closed for Helm:
 `deploy/helm/sentry/templates/api.yaml`/`enterprise-api.yaml` are
 mutually exclusive on the same `enterprise.enabled` flag that turns on