Co-authored-by: laipz8200 <16485841+laipz8200@users.noreply.github.com>

Signed-off-by: -LAN- <laipz8200@outlook.com>

Co-authored-by: 刘江波 <jiangbo721@163.com>

This PR addresses issue #21441 by implementing explicit `version` method definitions for all `BaseNode` subclasses to improve code maintainability.

### Changes

Added explicit `version` method definitions for all `BaseNode` subclasses:

- `QuestionClassifierNode`
- `KnowledgeRetrievalNode` 
- `AgentNode`

Added comprehensive test suite to validate:

1. All subclasses of `BaseNode` have explicitly defined `version` method
2. All subclasses have required `_node_type` property
3. The `(node_type, node_version)` combination is unique across all subclasses

Co-authored-by: QuantumGhost <2939865+QuantumGhost@users.noreply.github.com>

The frontend of feat: Persist Variables for Enhanced Debugging Workflow (#20699).

Co-authored-by: jZonG <jzongcode@gmail.com>

This pull request introduces a feature aimed at improving the debugging experience during workflow editing. With the addition of variable persistence, the system will automatically retain the output variables from previously executed nodes. These persisted variables can then be reused when debugging subsequent nodes, eliminating the need for repetitive manual input.

By streamlining this aspect of the workflow, the feature minimizes user errors and significantly reduces debugging effort, offering a smoother and more efficient experience.

Key highlights of this change:

- Automatic persistence of output variables for executed nodes.
- Reuse of persisted variables to simplify input steps for nodes requiring them (e.g., `code`, `template`, `variable_assigner`).
- Enhanced debugging experience with reduced friction.

Closes #19735.

Co-authored-by: lizb <lizb@sugon.com>

Co-authored-by: Xiaoba Yu <xb1823725853@gmail.com>

Co-authored-by: herui.gh <>

Co-authored-by: Xiaoba Yu <xb1823725853@gmail.com>

Signed-off-by: -LAN- <laipz8200@outlook.com>

Co-authored-by: zhuqingchao <zhuqingchao@xiaomi.com>