add explainTree command returning ExplainTreeNode for visual explain data

[shhhhreya]

Summary

Adds a new VS Code command vscode-db2i.explain.explainTree which constructs an ExplainTree from the provided vedata and returns the generated top-level ExplainTreeNode to get structured visual explain data.

Changes

• Registered new command: vscode-db2i.explain.explainTree
• Reuses existing ExplainTree logic to generate the tree structure
• Returns the top-level node via explainTree.get()
• No new logic or functionality was implemented.
• This change only exposes the existing logic through a newly registered command.

Why

This command is required for the BOB MCP tool to retrieve visual explain data in a proper structured format from vedata, so it can be rendered correctly in MCP tool.

[worksofliam]

I have explained this in other PRs, but we should be using a proper extension API for this functionality.

#480 (comment)

@SanjulaGanepola

[worksofliam]

See comment above

[SanjulaGanepola]

@shhhhreya As Liam mentioned, lets avoid adding arbitrary commands just for our extension to use. We should always look to add to the exported API as this is the standard way of exposing logic to other extensions. You can sorta see an example here: #480

In this case, we should look into what APIs we want to expose related to Visual Explain.
FYI, we may want to look into getting in #422 before doing this. I will help with this

[SanjulaGanepola]

You can find more info about exported APIs here: https://code.visualstudio.com/api/references/vscode-api#extensions. You can also refer to several of the other extensions to see how we structure this (check out Code for IBM i)