fix(coach): say so when the model answered with reasoning alone

When every provider in the chain answered a Coach round with reasoning and
no reply, even after being asked once more without reasoning, the turn ended
with "could not reach a provider, please try again in a moment". The
provider was reached, and trying again in a moment changes nothing for a
model that cannot stop reasoning.

Such a chain now ends with its own code, coach.provider.reasoning_only, and
the drawer says the model only reasoned and never wrote an answer, and
suggests another model. The choice between credential_expired, rate_limited,
reasoning_only and unavailable moved into one function beside the other turn
error rules, where it is tested on its own. An un-wrapped reasoning-only
error maps the same way. Clients that do not know the code keep showing the
generic provider message.

Refs #1276

src/components/insights/coach-panel/__tests__/message-thread.test.tsx

@@ -817,6 +817,14 @@ describe("errorCodeToI18nKey", () => {
     );
   });
 
+  // #1276 — a model that only reasoned was reached; "try again in a
+  // moment" is the wrong advice for it.
+  it("maps coach.provider.reasoning_only to its own copy", () => {
+    expect(errorCodeToI18nKey("coach.provider.reasoning_only")).toBe(
+      "insights.coach.errorReasoningOnly",
+    );
+  });
+
   // v1.39 — a refused turn names its reason. The capability refusals carry
   // their code in `meta.errorCode`; each maps to copy that says what is off
   // and who can change it, never the generic "provider unavailable".

src/components/insights/coach-panel/chat-bubble.tsx

@@ -87,6 +87,11 @@ export function errorCodeToI18nKey(code: string): string {
       // the user to guided setup with a one-line explainer + a Settings
       // link rendered alongside the bubble copy.
       return "insights.coach.errorNoProvider";
+    case "coach.provider.reasoning_only":
+      // #1276 — the model was reached and answered with its reasoning
+      // only, even after being asked once more. "Try again in a moment"
+      // would be the wrong advice; the copy names what happened.
+      return "insights.coach.errorReasoningOnly";
     case "coach.provider.unavailable":
     case "coach.provider.empty":
     case "coach.stream":