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.test.ts

@@ -68,6 +68,26 @@ describe("isContextLimitError", () => {
     expect(isContextLimitError("token limit reached for context window")).toBe(true);
   });
 
+  // ── Provider JSON envelope variants ────────────────────────────────
+
+  it("matches 'context window exceeds limit' variant from provider JSON envelope", () => {
+    // This is the specific error from the bug report:
+    // 400 {"type":"error","error":{"type":"invalid_request_error","message":"invalid params, context window exceeds limit (2013)"}}
+    expect(isContextLimitError("invalid params, context window exceeds limit (2013)")).toBe(true);
+  });
+
+  it("matches 'context window exceeds limit' without surrounding text", () => {
+    expect(isContextLimitError("context window exceeds limit")).toBe(true);
+  });
+
+  it("matches 'context window exceeds limit' with error code", () => {
+    expect(isContextLimitError("context window exceeds limit (2003)")).toBe(true);
+  });
+
+  it("matches 'context window exceeds' in mixed case", () => {
+    expect(isContextLimitError("Context Window Exceeds limit")).toBe(true);
+  });
+
   // ── Negative matches: must NOT trigger ─────────────────────────────
 
   it("returns false for empty string", () => {

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,
 ];
 
 /**