Add context menu support for branch collection nodes in SCM Repositories View

[Copilot]

Branch folder nodes (e.g., copilot/, benibenj/) in the SCM Repositories View had no context menu, preventing bulk operations on branches sharing a prefix.

Changes

SCM Workbench (src/vs/workbench/contrib/scm/)

• Extended onTreeContextMenu handler to support IResourceNode (folder nodes)
• Added collectChildArtifacts() to recursively collect leaf branch artifacts (childrenCount === 0)
• Added getArtifactFolderMenu() method to ISCMRepositoryMenus interface and implementation

Git Extension (extensions/git/)

• Added git.repositories.deleteBranches command with type-guarded array validation
• Sequential deletion with per-branch error logging
• Menu contribution to scm/artifact/context when scmArtifactGroupId == branches

Example

Right-clicking the copilot folder node now shows:

├── Checkout
├── Merge
├── Rebase
├── Create from...
├── Delete           ← existing (single branch)
├── Delete Branches  ← new (all child branches)
└── Compare with...

Selecting "Delete Branches" prompts: "Delete 5 branches?" and removes all branches under copilot/ prefix.

Fixes #<issue_number>

Original prompt

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This section details on the original issue you should resolve

<issue_title>SCM Repositories View: Support context menu actions on branch collections</issue_title>
<issue_description>I would like to be able to delete all branches with a specific prefix (for example copilot/) by using the context menu in the repositories view

</issue_description>

<agent_instructions>Add a context menu for branch nodes which have children. For example we render a node for benibenj when there is a branch called benibenj/testBranch. I want that benibenj node to have a context menu with the action delete. That action will delete all it's child branches. Related to the commandgit.repositories.deleteBranch which is handled in the artifactDeleteBranch method in extensions\git\src\commands.ts and is registered in the package.json of the git extension. Also related to onTreeContextMenu in src\vs\workbench\contrib\scm\browser\scmRepositoriesViewPane.ts</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes SCM Repositories View: Support context menu actions on branch collections #275788

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