gateway+validator: /v1/health reports honest worker count for production

Adds `fn len() -> usize` (default 0) to the WorkerLookup trait. The
InMemoryWorkerLookup overrides with HashMap size; ParquetWorkerLookup
constructs an InMemoryWorkerLookup so it inherits the count.

/v1/health now reports `workers_count` (exact integer) alongside
`workers_loaded` (derived bool: count > 0). The previous placeholder
true was a known caveat in the prior commit's body — this closes it.

Production switchover use case: J swaps workers_500k.parquet → real
Chicago contractor data, restarts the gateway, and verifies the
swap with one curl:

  curl http://localhost:3100/v1/health | jq .workers_count

Expected: matches the row count of the new file. Mismatch (or 0)
means the file is missing / unreadable / had a schema mismatch and
the gateway fell back to the empty InMemoryWorkerLookup. Operator
catches the drift before traffic reaches the validators.

Verified live (current synthetic data):
  workers_count: 500000   (matches workers_500k.parquet row count)
  workers_loaded: true

When the Chicago data lands, the same curl is the single source of
truth that the new dataset is hot. Removes the
restart-and-pray failure mode.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/gateway/src/v1/mod.rs

@@ -579,22 +579,12 @@ async fn usage(State(state): State<V1State>) -> impl IntoResponse {
 /// gates. A monitoring tool should evaluate the booleans + counts
 /// against its own thresholds.
 async fn health(State(state): State<V1State>) -> impl IntoResponse {
-    let workers_loaded = {
-        // Use a lookup with an obviously-fake id to probe — None
-        // could mean empty roster OR healthy roster without that id.
-        // We don't have a count() method on the trait; use a sample
-        // probe + treat presence of workers as a yes/no signal.
-        let probe = state.validate_workers.find("__healthcheck_probe__");
-        // probe is always None for the synthetic id, so this isn't
-        // useful. Better: rely on the fact that an empty-fallback
-        // InMemoryWorkerLookup ALSO returns None — there's no way
-        // to distinguish "loaded, just doesn't have this id" from
-        // "empty fallback". We'd need a count() method on WorkerLookup
-        // to report honestly. For now report the load attempt was
-        // performed (boot logs are the source of truth on rows count).
-        let _ = probe;
-        true
-    };
+    // Honest worker count via WorkerLookup::len. Production switchover
+    // verification: after swapping workers_500k.parquet → real Chicago
+    // data and restarting, this number should match the row count of
+    // the new file. 0 means the file was missing / unreadable / had a
+    // schema mismatch and the gateway booted with the empty fallback.
+    let workers_count = state.validate_workers.len();
     let providers_configured = serde_json::json!({
         "ollama_cloud": state.ollama_cloud_key.is_some(),
         "openrouter": state.openrouter_key.is_some(),
@@ -607,7 +597,8 @@ async fn health(State(state): State<V1State>) -> impl IntoResponse {
     let usage_snapshot = state.usage.read().await.clone();
     Json(serde_json::json!({
         "status": "ok",
-        "workers_loaded": workers_loaded,
+        "workers_count": workers_count,
+        "workers_loaded": workers_count > 0,
         "providers_configured": providers_configured,
         "langfuse_configured": langfuse_configured,
         "usage_total_requests": usage_snapshot.requests,