fix: detect context_overflow error for auto-compaction

Fixes #3154

Users hit context overflow errors that don't trigger auto-compaction because
isContextOverflowError() only checks for "context overflow" (space), but some
providers return "context_overflow" (underscore). Without this fix, the error
is displayed to the user instead of automatically triggering session compaction.

Changes:
- Add context_overflow (underscore) pattern to isContextOverflowError()
- Maintain backward compatibility with existing "context overflow" (space)
- Add test coverage for underscore format with 3 sample error messages
- Both patterns now trigger auto-compaction as configured

Reviewed-by: Codex (threadId: 019c036c-d6b7-7281-a654-93a2f1776997)
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/agents/pi-embedded-helpers.iscontextoverflowerror.test.ts

@@ -40,6 +40,20 @@ describe("isContextOverflowError", () => {
     }
   });
 
+  it("matches 'context_overflow' with underscore format", () => {
+    // Issue #3154: Some providers return "context_overflow" (underscore) instead of
+    // "context overflow" (space). Without this fix, auto-compaction is NOT triggered.
+    // The user sees the error instead of automatic compaction + retry.
+    const samples = [
+      "context_overflow: prompt too large for the model",
+      "Context_Overflow: request exceeds limit",
+      "ERROR: context_overflow detected",
+    ];
+    for (const sample of samples) {
+      expect(isContextOverflowError(sample)).toBe(true);
+    }
+  });
+
   it("ignores unrelated errors", () => {
     expect(isContextOverflowError("rate limit exceeded")).toBe(false);
     expect(isContextOverflowError("request size exceeds upload limit")).toBe(false);

src/agents/pi-embedded-helpers/errors.ts

@@ -21,6 +21,7 @@ export function isContextOverflowError(errorMessage?: string): boolean {
     lower.includes("exceeds model context window") ||
     (hasRequestSizeExceeds && hasContextWindow) ||
     lower.includes("context overflow") ||
+    lower.includes("context_overflow") ||
     (lower.includes("413") && lower.includes("too large"))
   );
 }