fix(ai): gate OpenAI JSON mode on responseFormat instead of forcing it

The OpenAI client pinned `response_format: { type: "json_object" }` on every
call. The Coach tool loop's forced-final round runs with `tool_choice: "none"`
and expects plain prose, but JSON mode coerced the reply into a JSON object —
the user saw a JSON blob or an empty reply instead of the coach's answer.

Send `response_format` only when the caller opts in with `responseFormat: "json"`
and no tools are present, mirroring the Anthropic client's prefill gate. The
JSON-consuming callers that relied on the implicit mode (insight generate +
briefing retry, medication extraction, the connection-test probe) now pass the
flag explicitly; the prose Coach path and every tool round stay out of JSON
mode. Codex already omitted the flag and is unaffected.

src/app/api/ai/test/route.ts

@@ -72,6 +72,9 @@ export const POST = apiHandler(async (request: NextRequest) => {
         user: 'Reply with the JSON object {"ok": true} and nothing else.',
         temperature: 0,
         maxTokens: 32,
+        // The probe asks for a JSON object — keep the OpenAI / Codex strict
+        // JSON mode it relied on before the response_format gate landed.
+        responseFormat: "json",
       }),
     );
 

src/app/api/insights/generate/route.ts

@@ -682,6 +682,9 @@ export const POST = apiHandler((request: NextRequest) =>
           user: userPrompt,
           temperature: 0.3,
           maxTokens: 1500,
+          // The reply is parsed with `JSON.parse` below, so opt the OpenAI /
+          // Codex chains into their strict JSON mode (gated on this flag).
+          responseFormat: "json",
         }),
       });
       result = fallback.result;
@@ -815,6 +818,8 @@ export const POST = apiHandler((request: NextRequest) =>
               user: `${userPrompt}\n\n${buildBriefingGroundingCorrection(ungrounded)}`,
               temperature: 0.3,
               maxTokens: 1500,
+              // Parsed with `JSON.parse` below — opt OpenAI / Codex into JSON mode.
+              responseFormat: "json",
             }),
           });
           const parsedRetry = JSON.parse(retry.result.content);

src/app/api/medications/extract/route.ts

@@ -195,6 +195,9 @@ async function handleExtract(request: NextRequest): Promise<Response> {
         // The reply body is a single small JSON object; 600 is well
         // over the empirical max we have seen on a 2k-char input.
         maxTokens: 600,
+        // Parsed with `parseModelJson` (JSON.parse) below — opt the OpenAI /
+        // Codex chains into their strict JSON-object mode (gated on this flag).
+        responseFormat: "json",
       }),
     });
   } catch (err) {