test(examples): let mock_file_source simulate an ingest failure via fail_start (#128)

The mock_file_source example doubles as the shared test fixture for PlotJuggler's
file-loader tests. Add a test-only failure-injection hook: when its config contains
the `fail_start` marker, importData() returns an error instead of writing rows.

This is needed because the host creates a dataset in the live engine BEFORE
importData() runs, so a source that loads/binds successfully but then fails during
ingest is the only way to exercise the host's abandoned-dataset cleanup path. The
existing deliberately-broken sibling plugins fail at discovery (before the dataset
exists), so they can't reach that path.

Scope: example/test plugin only — no installed header, no ABI, no canonical-object
change — so no SDK version bump (stays 0.8.1). config_ is matched by substring to
keep the example JSON-dependency-free; only tests set this config.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: facontidavide

pj_plugins/examples/mock_file_source.cpp

@@ -22,6 +22,15 @@ class MockFileSource : public PJ::FileSourceBase {
   }
 
   PJ::Status importData() override {
+    // Test-only failure injection: the host creates the dataset in the live engine
+    // before importData() runs, so returning an error here lets tests exercise the
+    // host's abandoned-dataset cleanup. config_ is opaque to this example (no JSON
+    // dependency), so the marker is matched by substring — safe because only tests
+    // set this config.
+    if (config_.find("fail_start") != std::string::npos) {
+      return PJ::unexpected("mock_file_source: configured start failure");
+    }
+
     if (!runtimeHost().progressStart("Importing", 3, true)) {
       return PJ::unexpected("progress unavailable");
     }