Make vscode.open agent-invocable

[dhruvisompura]

Addresses part of #15063.

Summary

Makes vscode.open invocable by Posit Assistant, so the assistant can open files -- including opening .csv/.parquet files in the Data Explorer via Positron's editor resolver.

The command already works through the agent path today: positron.ai.validateAndExecuteCommand executes the renderer-side registration directly (bypassing the extension-host API converter that rejects plain path strings), and a string path falls through to the opener service, which parses it as a file URI. The only gap was discovery metadata: the renderer registration in the upstream editorCommands.ts declared a bare Uri arg with no description or schema.

This PR adds agentCompatible: true and a documented string schema for the argument, wrapped in Start/End Positron markers with the upstream arg line preserved in a comment. No behavior change for any caller.

Screenshots

(to be added: assistant opening a csv in Data Explorer via positronCommand)

Release Notes

New Features

• Posit Assistant can open files, including tabular data files in the Data Explorer (Update core positron onboarding commands to be agent compatible #15063)

Bug Fixes

• N/A

Validation Steps

@:data-explorer @:editor

1. Verify the agent contract: run "Developer: Show All Agent-Compatible Commands". Expected: vscode.open listed with a described string Uri argument.
2. Verify user behavior is unchanged: open files normally (double-click in Explorer, Quick Open). Expected: no change.
3. Verify the headless path: in a workspace containing a csv, ask the assistant to "open data.csv". Expected: the file opens in the Data Explorer with no prompt.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical @:data-explorer @:editor

Why these tags?

Tag	Source
@:critical	Always runs (required)
@:data-explorer	PR description
@:editor	PR description

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