Fix map enumeration bug in DAG task filters + expand test coverage

[Copilot]

dag.tasks is a %{task_id => %DAGTask{}} map, but get_completed_tasks/1, get_pending_tasks/1, and get_running_tasks/1 were passing raw {key, value} tuples into DAGTask.is_completed/1 etc., causing FunctionClauseError at runtime on any non-empty DAG.

Bug Fix

# Before — crashes when tasks exist
Enum.filter(dag.tasks, fn task -> DAGTask.is_completed(task) end)

# After — correctly destructures map entries
Enum.filter(dag.tasks, fn {_task_id, task} -> DAGTask.is_completed(task) end)

New Test Files

• test/dag_task_test.exs — DAGTask.new/1, validate/1, status predicates, set_handler/2
• test/dag_run_test.exs — DAGRun.new/1, generate_id/0, random_string/1, uniqueness invariants
• test/dag_task_run_test.exs — DAGTaskRun.new/3, field initialization, payload storage, timestamps

Expanded ex_dag_test.exs

Covers the previously untested surface: get_completed_tasks/1, get_pending_tasks/1, get_running_tasks/1, get_tasks/1, set_handler/2, set_tasks_handler/2, validate_for_run/1, get_deps/2, get_deps_map/1, sorted_tasks/1, should_run_task/2, clear_failed_tasks_runs/1, completed?/1, String.Chars impl, and add_task!/2 throw behavior.

Original prompt

Pull Request: #5

Rebase on master

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