fix(core): detect context_length_exceeded in API call errors (backport B1 #17748)

Parse response body for error code so providers returning
context_length_exceeded are correctly classified as context overflow.

Upstream: anomalyco/opencode#17748 (e718db6)

Author: ghost91-

packages/opencode/src/provider/error.ts

@@ -167,7 +167,8 @@ export namespace ProviderError {
 
   export function parseAPICallError(input: { providerID: ProviderID; error: APICallError }): ParsedAPICallError {
     const m = message(input.providerID, input.error)
-    if (isOverflow(m) || input.error.statusCode === 413) {
+    const body = json(input.error.responseBody)
+    if (isOverflow(m) || input.error.statusCode === 413 || body?.error?.code === "context_length_exceeded") {
       return {
         type: "context_overflow",
         message: m,