feat(scm): 'Open File' in graph opens current file, add 'Open File at Commit'

[yogeshwaran-c]

Summary

Fixes #250938.

Previously the Open File toolbar button in the SCM Graph history item change context opened the historical (read-only) version of the file at that commit. When a user wanted to edit or view the current working-tree version of that file, they had to use Go to File and type the file name manually — a slow, all-manual routine.

This PR:

• Changes the existing Open File action to open the current (working tree) version of the file, matching what users expect from a button labeled "Open File"
• Adds a new Open File at Commit action (context menu only) to retain access to the historical read-only snapshot view

The historical diff view for a file is still shown when the user clicks the row in the graph, so the two entry points remain:

• Click row → read-only diff vs parent commit (existing behavior)
• Open File toolbar button → current working-tree file (new behavior)
• Open File at Commit context menu → historical snapshot (previous toolbar behavior)

Changes

• src/vs/workbench/contrib/scm/browser/scmHistoryViewPane.ts — updated the workbench.scm.action.graph.openFile action and added a new workbench.scm.action.graph.openFileAtCommit action

Test plan

• Open SCM Graph view, select a commit
• Click Open File toolbar button on a file — verify it opens the current working-tree version
• Right-click the file → Open File at Commit — verify it opens the historical snapshot with (commit-id) label

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@lszomoru

Matched files:

• src/vs/workbench/contrib/scm/browser/scmHistoryViewPane.ts