test: add coverage for graph queries and parser error paths (6 new tests)

[github-actions]

What gap was identified

Two areas with zero test coverage were identified:

1. Graph query methods — src/graph/queries.rs has methods find_action_invokers, find_usages, find_variable_writers, get_topic_reasoning_actions, and get_topic_action_defs with no tests at all. The existing query tests only covered find_outgoing_transitions, find_incoming_transitions, topic_execution_order, and stats.

2. Parser error paths — none of the test files contained a test that expected parse_source to return Err. All existing tests verified successful parses, leaving the error-reporting code paths untested.

Files modified

• src/graph/queries.rs — 5 new tests added to the existing #[cfg(test)] mod tests block
• tests/integration_test.rs — 1 new test added to the existing mod parser_unit_tests block

New tests

Test	What it covers
test_find_action_invokers_returns_invoking_reasoning_action	A reasoning action targeting @actions.lookup creates an Invokes edge; find_action_invokers(lookup_def_idx) must return that reasoning action
test_get_topic_reasoning_actions_counts_correctly	get_topic_reasoning_actions("topic_a") returns exactly the 1 reasoning action in that topic; returns empty for a leaf topic and for an unknown topic name
test_get_topic_action_defs_counts_correctly	get_topic_action_defs("main") returns exactly 2 defs for a topic with two action definitions; returns empty for an unknown topic
test_find_variable_writers_after_set_clause	A reasoning action with set @variables.step = 2 creates a Writes edge; find_variable_writers(step_idx) must include that reasoning action
test_find_usages_returns_all_incoming_edges_for_topic	find_usages(topic_b) returns all incoming edges (not filtered by type), distinguishing it from find_incoming_transitions
test_parse_error_on_unknown_variable_type	parse_source with an unrecognized type keyword (notavalidtype) must return Err — exercises the error-reporting path

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