fix(migration): treat an unselected runner id as not_legacy

A saved pipeline may carry the post-migration shape
{'ai': {'runner': {'id': ''}, 'runner_config': {}}} while no runner has been
selected yet. The planner matched the empty id against the certified
'plugin:author/name/component' form, failed, and returned a malformed-id
blocker, so the whole 'migrate all' batch stopped with both pipelines reported
as blocked even though there was no legacy section and nothing to convert.

A blank id with no legacy runner section now reports not_legacy: no work is
needed and no target is synthesized. A blank id that still coexists with a
legacy section remains an explicit invalid_runner_id blocker, because that
ambiguous state must be resolved by the operator rather than guessed.

src/langbot/pkg/pipeline/legacy_config_migration.py

@@ -634,6 +634,12 @@ def plan_legacy_pipeline(config, extensions_preferences=None) -> dict:
         return _block(result, 'mixed_runner_selection', 'ai.runner')
     if 'id' in selection:
         current = selection['id']
+        # A blank runner id paired with no legacy runner section means the saved
+        # pipeline simply has no runner selected. There is nothing to migrate and
+        # nothing to convert, so report not_legacy instead of failing the whole
+        # batch with a malformed-id blocker.
+        if type(current) is str and not current.strip() and not any(legacy in ai for legacy in _TARGETS):
+            return result
         if type(current) is not str or not re.fullmatch(r'plugin:[^/\s]+/[^/\s]+/[^/\s]+', current):
             return _block(result, 'invalid_runner_id', 'ai.runner.id')
         result['state'] = 'already_current'

tests/unit_tests/pipeline/test_legacy_config_migration.py

@@ -641,12 +641,40 @@ def test_dify_saved_timeout_is_not_blindly_activated(timeout):
     assert {'code': 'dify.timeout_default', 'field': 'ai.dify-service-api.timeout'} in result['warnings']
 
 
-@pytest.mark.parametrize('current', [None, '', False, [], 'plugin:bad', ' plugin:a/b/c', 'plugin:a/b/c/extra'])
+@pytest.mark.parametrize('current', [None, False, [], 'plugin:bad', ' plugin:a/b/c', 'plugin:a/b/c/extra'])
 def test_malformed_current_id_is_not_already_current(current):
     source = {'ai': {'runner': {'id': current}}}
     assert_block(plan(source), 'invalid_runner_id', 'ai.runner.id')
 
 
+@pytest.mark.parametrize('blank', ['', '   '])
+def test_blank_current_id_without_legacy_section_is_not_legacy(blank):
+    """A saved pipeline that simply has no runner selected is not legacy.
+
+    It has no legacy runner section to convert and no target to synthesize, so it
+    must report not_legacy instead of blocking the whole batch with a
+    malformed-id diagnostic.
+    """
+    source = {'ai': {'runner': {'id': blank, 'expire-time': 0}, 'runner_config': {}}}
+    result = plan(source)
+    assert result['state'] == 'not_legacy'
+    assert result['config'] is None
+    assert result['blockers'] == []
+    assert result['changed_paths'] == []
+    assert result['target_plugin'] is None
+
+
+def test_blank_current_id_with_legacy_section_stays_blocked():
+    """A blank id cannot silently coexist with a legacy section.
+
+    An unselected plugin runner plus a legacy section is an ambiguous state that
+    the operator must resolve, so it keeps the malformed-id blocker.
+    """
+    source = source_for()
+    source['ai']['runner'] = {'id': '', 'expire-time': 0}
+    assert_block(plan(source), 'invalid_runner_id', 'ai.runner.id')
+
+
 @pytest.mark.parametrize('runner', TARGETS)
 def test_deterministic_results_and_input_nonmutation_for_all_nine(runner):
     source = source_for(runner)