Fix child startup readiness and ordered provider tools

Wait for shell bootstrap before dispatching child prompts, serialize provider preprocessing, and make permission callbacks idempotent. Restrict task lists to concrete multistep plans and stop inferring completion from tool activity.

Update regression fixtures and resolve existing lint and test-layout failures. Verified formatting, both presubmit Clippy commands, and 354 targeted nextest tests.

app/src/ai/provider/discovery_tests.rs

@@ -0,0 +1,35 @@
+use super::model_availability_is_usable;
+
+#[test]
+fn requires_every_availability_status() {
+    assert!(model_availability_is_usable(
+        Some("AVAILABLE"),
+        "AUTHORIZED",
+        "AVAILABLE",
+        "AVAILABLE",
+    ));
+    assert!(!model_availability_is_usable(
+        None,
+        "AUTHORIZED",
+        "AVAILABLE",
+        "AVAILABLE",
+    ));
+    assert!(!model_availability_is_usable(
+        Some("AVAILABLE"),
+        "NOT_AUTHORIZED",
+        "AVAILABLE",
+        "AVAILABLE",
+    ));
+    assert!(!model_availability_is_usable(
+        Some("AVAILABLE"),
+        "AUTHORIZED",
+        "NOT_AVAILABLE",
+        "AVAILABLE",
+    ));
+    assert!(!model_availability_is_usable(
+        Some("AVAILABLE"),
+        "AUTHORIZED",
+        "AVAILABLE",
+        "NOT_AVAILABLE",
+    ));
+}