refactor(core): Initial support for node-hierarchy in the execution engine

[netroy]

Summary

This PR introduces a new execution context type that's used for AI root nodes like Agents to build a hierarchy of node executions in the execution engine.

Related Linear tickets, Github issues, and Community forum posts

Review / Merge checklist

• PR title and summary are descriptive. (conventions)
• Docs updated or follow-up ticket created.
• Tests included.
• PR Labeled with release/backport (if the PR is an urgent fix that needs to be backported)

[codecov]

Codecov Report

Attention: Patch coverage is 35.36585% with 106 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...kages/@n8n/nodes-langchain/nodes/code/Code.node.ts	0.00%	17 Missing ⚠️
...ages/@n8n/nodes-langchain/utils/N8nBinaryLoader.ts	0.00%	7 Missing ⚠️
...s-langchain/nodes/chains/ChainLLM/ChainLlm.node.ts	0.00%	6 Missing ⚠️
...c/node-execution-context/node-execution-context.ts	70.00%	6 Missing ⚠️
...in/nodes/agents/Agent/agents/ToolsAgent/execute.ts	0.00%	5 Missing ⚠️
...gents/Agent/agents/OpenAiFunctionsAgent/execute.ts	0.00%	4 Missing ⚠️
...agents/Agent/agents/ConversationalAgent/execute.ts	0.00%	3 Missing ⚠️
...ChainSummarization/V2/ChainSummarizationV2.node.ts	0.00%	3 Missing ⚠️
...odes/tools/ToolVectorStore/ToolVectorStore.node.ts	0.00%	3 Missing ⚠️
...nodes/vector_store/shared/createVectorStoreNode.ts	0.00%	3 Missing ⚠️
... and 33 more

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