test: add coverage for parser error paths and graph queries (7 new tests)

[github-actions]

Gap identified

Two areas had zero test coverage:

1. Parser error paths — no existing test ever asserts that parse() returns Err. All tests only cover valid input. The validator in variables.rs explicitly emits errors for = None on non-boolean types, and the config parser rejects non-string values, but neither was exercised.

2. Graph query functions — find_action_invokers, find_variable_readers, and find_variable_writers in src/graph/queries.rs had no unit tests, despite being public API used by graph analysis features.

Files modified

File	Change
tests/test_parser_errors.rs	New integration test file (5 tests)
src/graph/queries.rs	2 new tests in existing #[cfg(test)] mod tests block
Cargo.toml	Added [[test]] entry for test_parser_errors

New tests

tests/test_parser_errors.rs

Test	What it covers
test_none_default_for_string_variable_is_rejected	Validator in variables.rs rejects = None for mutable string
test_none_default_for_integer_variable_is_rejected	Same validator rejects = None for mutable integer
test_none_default_for_boolean_variable_is_valid	Confirms the validator allows = None for mutable boolean (valid case)
test_config_field_value_must_be_a_string_literal	Config parser rejects a bare number (42) where a string literal is required
test_empty_file_parses_to_default_agent	Empty input is syntactically valid, producing an AgentFile with all fields absent

src/graph/queries.rs

Test	What it covers
test_find_action_invokers_returns_correct_node	A reasoning action pointing to @actions.get_status creates a RefEdge::Invokes edge; find_action_invokers(action_def) returns that reasoning action
test_find_variable_writers_from_set_clause	A set @variables.x = @outputs.y clause creates a RefEdge::Writes edge; find_variable_writers(var) returns the reasoning action that writes it

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