fix: surface a clear error on non-JSON API responses instead of crashing

When a proxy, gateway or CDN returns an HTTP 200 with an HTML body (an error
or interstitial page) instead of JSON, the generated SDK client JSON-parses it
and throws a raw `JSON Parse error: Unrecognized token '<'`. `parseAs` falls
back to "json" whenever Content-Type is missing or unrecognized, so a non-JSON
body reaches the parser. The error path was already guarded; the success path
was not.

Guard the JSON parse in both the v1 and v2 generated clients: on a parse
failure, throw an actionable error (non-JSON response, likely a proxy/gateway
error page, with HTTP status + content-type) instead of the raw parse crash.

Surfaced from telemetry as a recurring extension sendMessageError.

Author: ralphstodomingo

packages/sdk/js/src/gen/client/client.gen.ts

@@ -115,6 +115,20 @@ export const createClient = (config: Config = {}): Client => {
         case "blob":
         case "formData":
         case "json":
+          try {
+            data = await response.json()
+          } catch {
+            // A 200 with a non-JSON body — typically an HTML error / interstitial
+            // page from a proxy, gateway or CDN, not the API. `parseAs` falls back
+            // to "json" when Content-Type is missing/unrecognized, so this reaches
+            // JSON parsing. Surface an actionable error instead of a raw
+            // "Unrecognized token '<'" crash.
+            throw new Error(
+              `Expected a JSON response but received ${response.headers.get("content-type") || "an unknown content type"} ` +
+                `(HTTP ${response.status}). This is usually a proxy or gateway error page, not the API.`,
+            )
+          }
+          break
         case "text":
           data = await response[parseAs]()
           break

packages/sdk/js/src/v2/gen/client/client.gen.ts

@@ -169,7 +169,19 @@ export const createClient = (config: Config = {}): Client => {
           // Some servers return 200 with no Content-Length and empty body.
           // response.json() would throw; read as text and parse if non-empty.
           const text = await response.text()
-          data = text ? JSON.parse(text) : {}
+          try {
+            data = text ? JSON.parse(text) : {}
+          } catch {
+            // A 200 with a non-JSON body — typically an HTML error / interstitial
+            // page from a proxy, gateway or CDN, not the API. `parseAs` falls back
+            // to "json" when Content-Type is missing/unrecognized, so this reaches
+            // JSON.parse. Surface an actionable error instead of a raw
+            // "Unrecognized token '<'" crash.
+            throw new Error(
+              `Expected a JSON response but received ${response.headers.get("content-type") || "an unknown content type"} ` +
+                `(HTTP ${response.status}). This is usually a proxy or gateway error page, not the API.`,
+            )
+          }
           break
         }
         case "stream":