feat(FN-1462): add context window exceeds limit pattern to context-limit-detector

- Add new regex pattern /context\s+window\s+exceeds/i to match provider JSON error
  envelope variant: "context window exceeds limit (2013)"
- Add 4 new test cases for the variant pattern
- Pattern is conservative: requires both "context window" and "exceeds" present

packages/engine/src/context-limit-detector.ts

@@ -39,6 +39,9 @@ const CONTEXT_OVERFLOW_PATTERNS: RegExp[] = [
   /too many tokens/i,
   // Anthropic variant: "messages with that many tokens would exceed"
   /tokens? would exceed/i,
+  // Provider JSON error envelope variant: "context window exceeds limit (2013)"
+  // Matches when "context window" and "exceeds" appear together (order-flexible)
+  /context\s+window\s+exceeds/i,
 ];
 
 /**