Fix git repository cache to skip untitled workspaces

[Copilot]

Opening a PR from a URI handler link after VS Code restart showed "Path does not exist" error with a numeric path like \1761808101585.

Root cause: RepositoryCache._findWorkspaceForRepo called .fsPath on workspace.workspaceFile without checking its scheme. For untitled workspaces (untitled:1761808101585), .fsPath returns just the numeric timestamp ID. This invalid path was cached and later passed to Uri.file(), creating file:///1761808101585.

Changes:

• Add scheme check before using workspaceFile.fsPath - only use workspace files with file scheme
• For untitled workspaces, fall back to workspace folders which have valid paths
• Add tests for untitled workspace handling and file-scheme workspace behavior

// Before: used any workspaceFile
if (this._workspaceFile) {
    folderPathOrWorkspaceFile = this._workspaceFile.fsPath;
}

// After: only use file-scheme workspaces
if (this._workspaceFile && this._workspaceFile.scheme === 'file') {
    folderPathOrWorkspaceFile = this._workspaceFile.fsPath;
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Error message on second open of PR from a link</issue_title>
<issue_description>Testing #274805

Repro Steps

1. Enlist and load vscode repo
2. From browser: open PR: vscode-insiders://github.vscode-pull-request-github/checkout-pull-request?%7B%22owner%22%3A%22microsoft%22%2C%22repo%22%3A%22vscode%22%2C%22pullRequestNumber%22%3A273860%7D
3. Once complete, switch to Explorer View, close all other windows
4. Close VS Code
5. Reopen VS Code -> this will show empty env with vscode repo loaded in explorer
6. From browser: open PR again: vscode-insiders://github.vscode-pull-request-github/checkout-pull-request?%7B%22owner%22%3A%22microsoft%22%2C%22repo%22%3A%22vscode%22%2C%22pullRequestNumber%22%3A273860%7D

Actual Result

Expected Result
No Error</issue_description>

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

@dmitrivMS NOTE: If you open PR explorer pane first, it does not seem to happen. Logs: ``` 2025-11-04 14:07:50.691 [info] [Activation] Extension version: 0.121.2025110404 2025-11-04 14:07:50.704 [info] [Authentication] Creating hub for .com 2025-11-04 14:07:50.903 [info] [Activation] Looking for git repository 2025-11-04 14:07:50.903 [info] [Activation] Found 0 repositories during activation 2025-11-04 14:07:50.903 [info] [Activation] Git repository found, initializing review manager and pr tree view. 2025-11-04 14:07:50.904 [info] [GitAPI] Registering git provider 2025-11-04 14:07:50.918 [info] [GitAPI] Repository file:///c%3A/Users/dmitriv/repos/vscode has been opened 2025-11-04 14:07:54.989 [info] [Review+0] Validate state in progress 2025-11-04 14:07:54.989 [info] [Review+0] Validating state... 2025-11-04 14:07:54.989 [info] [FolderRepositoryManager+0] Cannot updates repositories as git is uninitialized 2025-11-04 14:07:54.989 [info] [Activation] Repo state for file:///c%3A/Users/dmitriv/repos/vscode changed. 2025-11-04 14:07:54.989 [info] [Activation] Repo file:///c%3A/Users/dmitriv/repos/vscode has already been setup. 2025-11-04 14:07:55.869 [info] [Activation] Git initialization state changed: state=initialized 2025-11-04 14:07:55.869 [info] [Review+0] Validate state in progress 2025-11-04 14:07:55.869 [info] [Review+0] Validating state... 2025-11-04 14:07:55.871 [info] [FolderRepositoryManager+0] Found GitHub remote for folder c:\Users\dmitriv\repos\vscode 2025-11-04 14:07:56.325 [info] [FolderRepositoryManager+0] Trying to use globalState for assignableUsers. 2025-11-04 14:07:56.421 [info] [Review+0] Found matching pull request metadata for current branch dev/dmitriv/glob-2. Repo: microsoft/vscode PR: 273860 2025-11-04 14:07:56.421 [info] [Review+0] current branch dev/dmitriv/glob-2 is associated with pull request #273860 2025-11-04 14:07:56.421 [info] [Review+0] Resolving pull request 2025-11-04 14:07:56.439 [info] [FolderRepositoryManager+0] Using globalState assignableUsers for 1. 2025-11-04 14:07:57.148 [info] [FolderRepositoryManager+0] Using in-memory cached assignable users. 2025-11-04 14:07:57.699 [info] [GitHubRepository+0] Fetch branch main - enter 2025-11-04 14:07:58.140 [info] [GitHubRepository+0] Fetch branch main - done: true 2025-11-04 14:07:58.148 [info] [Review+0] Resolved PR #273860, state is OPEN 2025-11-04 14:07:58.148 [info] [Review+0] Fetching pull request data 2025-11-04 14:07:58.148 [info] [PullRequestTree] Adding PR #273860 to tree 2025-11-04 14:07:58.167 [info] [Review+0] Register comments provider 2025-11-04 14:07:58.168 [info] [Review+0] Register in memory content provider 2025-11-04 14:07:59.175 [info] [Review+0] Display pull request status bar indicator. 2025-11-04 14:07:59.175 [info] [Review+0] Using focus mode = true. 2025-11-04 14:07:59.175 [info] [Review+0] State validation silent = true. 2025-11-04 14:07:59.175 [info] [Review+0] PR show should show = false. ``` @alexr00 I haven't been able to repro this, and it's strange because where that error comes from there must be a `file` scheme URI, which points to the workspace we're trying to open being incorrect. Adding some logging. @alexr00 New pre-release is building now. @dmitrivMS, could you please install the new pre-release once it's available, repro the bug, then share the output of GitHub Pull Request? @dmitrivMS @alexr00 Still repros.

One correction to the repro steps above (not sure if it matters): I am opening a PR for which I have a local branch and it is ...

• Fixes Error message on second open of PR from a link #275356

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