Preserve unspecified node params on partial updates (recursive merge)

[zhongkaifu]

Motivation

• Update operations should not drop or overwrite node params fields that the caller did not specify, especially for nested structures.
• Existing behavior could remove nested keys when performing partial updates; this change ensures partial/patch semantics for node updates.

Description

• Added helper functions _deep_merge_mapping and _merge_node_params to recursively merge incoming params onto existing node params while preserving unspecified fields.
• Applied the merge step before schema filtering/validation in all update tools: update_action_node, update_reasoning_node, update_data_node, update_condition_node, update_switch_node, and update_loop_node by using merged_params as the input to filter_supported_params.
• Adjusted condition-specific handling to merge and validate params.expression against the merged params.
• Added a regression test tests/test_structure_update_node_partial_params.py that simulates an update_action_node call updating only a nested field and asserts other nested fields remain unchanged.

Testing

• Ran pytest -q tests/test_structure_update_node_partial_params.py tests/test_workflow_builder_update.py, and the test suite completed successfully (all tests passed).

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Codex Task