docs(bf-3qpih): verify passing test case exists for assert_exit_code

Add verification note documenting that the passing test case for
assert_exit_code already exists in the codebase and passes all acceptance
criteria.

Test: test_extraction_result_assert_exit_code_success passes
File: crates/pdftract-core/src/extract.rs:3070-3099
Verification: notes/bf-3qpih.md

notes/bf-3qpih.md

@@ -0,0 +1,96 @@
+# bf-3qpih: Write passing test case for assert_exit_code
+
+## Summary
+
+Verified that the passing test case for `assert_exit_code` already exists in the codebase at `/home/coding/pdftract/crates/pdftract-core/src/extract.rs:3070-3099`.
+
+## Test Details
+
+**Test Name:** `test_extraction_result_assert_exit_code_success`
+
+**Location:** `crates/pdftract-core/src/extract.rs:3070-3099`
+
+**Test Logic:**
+1. Creates an `ExtractionResult` with `error_count = 0` (which corresponds to exit code 0)
+2. Calls `assert_exit_code(0)` 
+3. Asserts that the result is `Ok(())` using `assert!(result.assert_exit_code(0).is_ok())`
+
+**Test Code:**
+```rust
+#[test]
+fn test_extraction_result_assert_exit_code_success() {
+    // Test that assert_exit_code returns Ok(()) when exit codes match
+    let result = ExtractionResult {
+        fingerprint: "test".to_string(),
+        pages: vec![],
+        metadata: ExtractionMetadata {
+            page_count: 0,
+            receipts_mode: ReceiptsMode::Off,
+            span_count: 0,
+            block_count: 0,
+            cache_status: None,
+            cache_age_seconds: None,
+            error_count: 0,
+            reading_order_algorithm: None,
+            diagnostics: vec![],
+            profile_name: None,
+            profile_version: None,
+            profile_fields: None,
+        },
+        signatures: vec![],
+        form_fields: vec![],
+        links: vec![],
+        attachments: vec![],
+        threads: vec![],
+        javascript_actions: vec![],
+    };
+
+    // Should succeed - extraction with no errors should have exit code 0
+    assert!(result.assert_exit_code(0).is_ok());
+}
+```
+
+## Verification
+
+Ran the test to confirm it passes:
+```bash
+cargo nextest run -p pdftract-core test_extraction_result_assert_exit_code_success --no-capture
+```
+
+**Result:** ✓ PASS
+
+```
+running 1 test
+test extract::tests::test_extraction_result_assert_exit_code_success ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 2902 filtered out; finished in 0.00s
+```
+
+## Acceptance Criteria Status
+
+- [x] Test case exists for passing scenario
+- [x] Test creates ExtractionResult with exit code 0 (via error_count = 0)
+- [x] Test calls assert_exit_code(0) and expects Ok(())
+- [x] Test passes when run
+- [x] Test follows existing test patterns in the codebase
+
+## Additional Context
+
+The test follows the standard pattern for assertion method tests in the codebase:
+- Uses `#[test]` attribute
+- Includes descriptive comment explaining what is being tested
+- Creates test fixtures inline with struct literal syntax
+- Uses `assert!(...)` macro for simple boolean assertions
+- Follows naming convention: `test_<module>_<method>_<scenario>`
+
+Related tests that also exist:
+- `test_extraction_result_assert_exit_code_mismatch` - tests failure case
+- `test_extraction_result_assert_exit_code_with_errors` - tests exit code 1 for extractions with errors
+
+## Implementation Method
+
+The `assert_exit_code` method implementation is at `/home/coding/pdftract/crates/pdftract-core/src/extract.rs:2719-2736`. It computes the exit code based on `error_count` (0 for success, 1 for any errors) and returns `Ok(())` when the actual exit code matches the expected value.
+
+## Conclusion
+
+All acceptance criteria are met. The passing test case for `assert_exit_code` is already implemented and functional.