fix(api): provider-chain GET preserves disabled entries (H2 code-review)

The GET response used to surface the resolveProviderChain()-filtered
list, which DROPS disabled entries upstream. The Settings UI then
seeded `enabled: true` for every returned row, so disabling an entry
made it disappear on next refetch and lose its toggle position —
the toggle was functionally a "remove from chain" button.

Changed GET to read the *persisted* chain via parseProviderChain() so
disabled rows still round-trip with their flag. The active-provider
summary still uses resolveProviderChain so the displayed "Active: …"
line continues to reflect the actual runner ordering (enabled-only).
The Settings UI seeds entries with the wire-supplied `enabled` value
instead of always-true, so re-enabling becomes possible without a
page reload.

4 unit tests pin the GET shape (persisted-chain ordering, disabled
entries surfaced, cached-active overlay, default-chain fallback).

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

src/components/settings/ai-section.tsx

@@ -83,7 +83,11 @@ interface ChainEntry {
 interface ProviderChainData {
   activeProvider: ProviderType | null;
   cachedActiveProvider: ProviderType | null;
-  configuredChain: { providerType: ProviderType; available: boolean }[];
+  configuredChain: {
+    providerType: ProviderType;
+    enabled: boolean;
+    available: boolean;
+  }[];
 }
 
 const DEFAULT_CHAIN: readonly ChainEntry[] = [
@@ -1257,7 +1261,11 @@ function FallbackChainCard({
   selected,
   onSelect,
 }: {
-  chain: { providerType: ProviderType; available: boolean }[];
+  chain: {
+    providerType: ProviderType;
+    enabled: boolean;
+    available: boolean;
+  }[];
   selected: ProviderType;
   onSelect: (next: ProviderType) => void;
 }) {
@@ -1267,17 +1275,22 @@ function FallbackChainCard({
   // Local working copy. Server-confirmed values arrive via `chain`
   // prop; we keep our own state so the user can shuffle multiple rows
   // before clicking "Save chain order".
-  const seededKey = chain.map((c) => c.providerType).join(",");
+  // v1.4.16 phase D reconcile (code-review H2) — `enabled` from the
+  // wire is now the canonical state. The GET endpoint surfaces the
+  // raw persisted chain so a disabled entry survives the round-trip.
+  const seededKey = chain
+    .map((c) => `${c.providerType}:${c.enabled ? 1 : 0}`)
+    .join(",");
   const [seeded, setSeeded] = useState<string | null>(null);
   const [entries, setEntries] = useState<ChainEntry[]>(() =>
-    chain.map((c) => ({ providerType: c.providerType, enabled: true })),
+    chain.map((c) => ({ providerType: c.providerType, enabled: c.enabled })),
   );
   if (seededKey !== seeded) {
     setSeeded(seededKey);
     setEntries(
       chain.map((c) => ({
         providerType: c.providerType,
-        enabled: true,
+        enabled: c.enabled,
       })),
     );
   }