refactor: Migrate existing nodes (Function/Tool/start + test nodes) to new Node interface

[hanorik]

This PR updates the workflow system's node architecture to adopt the new Node interface, delegating schema routing and config management to BaseNode, and consolidating validation in the scheduler.

Key Changes

• Production Nodes Migration: Updated *FunctionNode, *ToolNode, and *startNode to satisfy the new Node interface by routing schemas and configurations through BaseNode.
• Test Nodes Migration: Refactored all 10 test nodes (e.g., dummyNode, cancelObservingNode, retryTestNode, etc.) to satisfy the new interface, primarily via BaseNode embedding.
• Consolidated Schema Validation: Removed lingering/redundant validation and conversion logic in execution paths (wrappedFn and runTool paths in function_node.go and tool_node.go). Input validation is now exclusively handled by the scheduler.
• Test Alignment: Updated test suites (e.g., tool_node_test.go and agent_node_test.go) to accommodate new constructors and ensure input validation is invoked prior to running nodes where required.

Verification

• All tests in the workflow package pass successfully:

  go test ./workflow/...

[wolo-lab]

I think keeping it doesn't have drawbacks and makes the code more user friendly (e.g. no need to change workflow/agent_node_test.go and you can revert changes in TestToolNode_WorkflowIntegration at workflow/tool_node_test.go: