Add "Show Help at Cursor" to the editor context menu

[juliasilge]

Fixes #14570

This PR adds "Show Help at Cursor" to the editor right-click context menu. The command already existed with keybindings (F1, Cmd+K Cmd+H) and a command palette entry; this PR just surfaces it in the context menu where users might look for it, as requested in the issue.

This is purely declarative metadata. A menu entry added to the existing ShowHelpAtCursor action registration. There is no new runtime logic, and the action's run() method is unchanged.

The when clause reuses the POSITRON_RUNTIME_LANGUAGE_IDS context key I added in #13098 (for "View Data Frame at Cursor"), so the item shows on editors whose language has a registered Positron runtime, plus .qmd explicitly (the embedded language is resolved at the cursor). The entry is ordered to sit just below "Go to References" and just above "View Data Frame at Cursor" in the navigation group:

Release Notes

New Features

• Added "Show Help at Cursor" to the editor context menu (Add "Help: Show Help at Cursor" to right click context menu  #14570).

Bug Fixes

• N/A

QA Notes

@:help @:editor @:quarto

There are no automated tests here because the change is declarative menu registration with no new logic to exercise. We can verify manually during review:

1. In an R file, right-click on a function name (e.g. library). Confirm "Show Help at Cursor" appears in the context menu, directly below "Go to References" and above "View Data Frame at Cursor". Choosing it opens the help topic.
2. Repeat in a Python file (e.g. right-click on print).
3. In a .qmd file with an R chunk or a Python chunk, place the cursor inside each chunk on a symbol and confirm the item appears and opens help for the correct language (this exercises the embedded-language path).
4. In a plaintext or Markdown file with no runtime language, confirm the item does NOT appear.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical @:help @:editor @:quarto

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Tag	Source
@:critical	Always runs (required)
@:help	PR description
@:editor	PR description
@:quarto	PR description

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[austin3dickey]

Works well for me, thanks!