Fix local history commands when resource URI scheme is vscode-local-history (#306147)

* Initial plan

* fix: map vscode-local-history URIs back to original file URI in findLocalHistoryEntry

When the diff editor shows two local history versions, the active editor
URI uses the vscode-local-history scheme. findLocalHistoryEntry now converts
this back to the original file URI before querying workingCopyHistoryService,
matching the existing logic in provideTimeline.

Fixes microsoft/vscode#250469

Agent-Logs-Url: https://github.com/microsoft/vscode/sessions/aae53613-4b18-418c-867c-d650153ddbbf

Co-authored-by: bpasero <[email protected]>

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: bpasero <[email protected]>