[v3-1-test] Fix grid view crash when task converted to TaskGroup (#61208) (#61279)

[pierrejeambrun]

• Fix grid view crash when task converted to TaskGroup (Broken grid view #61208)

This commit fixes a TypeError crash in the grid view endpoint when a task is converted to a TaskGroup (or vice versa) between DAG versions.

Root Cause:

• Old DagRuns had task structure with children=None
• New DagRuns had TaskGroup structure with children=[...]
• The _merge_node_dicts function tried to iterate over None -> TypeError

Changes:

1. Added defensive None checks in _merge_node_dicts function
2. Only merge children if both nodes have children (not None)
3. Added comprehensive unit tests for edge cases
4. Added integration test for task->TaskGroup conversion scenario

Fixes #61208

• Fix merge logic and add comprehensive tests per reviewer feedback

• Fix integration test: use SerializedDagModel.write_dag instead of DBDagBag.bag_dag

• Fix import: use serialized_objects.LazyDeserializedDAG instead of definitions.dag

• Simplify test: use only sync_dag_to_db (removes redundant write_dag calls)

• Trigger CI re-run

• Fix test case: invert v1/v2 to test TaskGroup-to-task conversion

The bug in issue #61208 occurs when a TaskGroup is converted to a simple task, not the other way around. This commit inverts Version 1 and Version 2 in the test case to properly test the actual bug scenario:

• Version 1: task_a is a TaskGroup with subtasks
• Version 2: task_a becomes a simple task

Updated all comments and assertions accordingly.

• Fix trailing blank line in test_grid.py

Pre-commit hooks removed trailing blank line - updating to match.

• Improve test assertions with full expected JSON

• Add API call after v1 to verify TaskGroup structure
• Replace partial assertions with full expected JSON comparison
• Verify v1 shows TaskGroup with children, v2 shows simple task
• Addresses review feedback from @jason810496

• Adjust and clean test

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(cherry picked from commit 060532b)

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[pierrejeambrun]

Backport of #62181