Fix darc vmr cherry-pick to honor --vmr and improve error messaging

[Copilot]

darc vmr cherry-pick was silently ignoring the --vmr option and always applying patches to the current directory's git root, making it impossible to cherry-pick without being physically inside the VMR directory.

Root cause

CherryPickOperation used Environment.CurrentDirectory exclusively for VMR detection and patch application, never consulting _options.VmrPath. This caused two bugs:

• --vmr /path/to/vmr was a no-op in the operation logic
• Patches were always applied to the current dir, so running from a repo dir would apply changes to the repo instead of the VMR

Changes

• VMR detection: Check _options.VmrPath (from --vmr, defaults to current dir) for source-manifest.json first. Fall back to checking --source as the VMR for backward compatibility (old pattern: run from repo, --source = VMR path).
• Patch application: Apply patches to vmrPath when flowing repo→VMR, or repoPath when flowing VMR→repo — instead of always targeting the current dir's git root.
• Error message: When neither path resolves to a VMR, emit a clear, actionable error:

  Could not find a VMR at 'C:\r\runtime' (missing 'C:\r\runtime\src\source-manifest.json').
  Run the operation from the VMR directory or specify the VMR path with --vmr.
  

• Logging: Replace the ambiguous "starting from VMR/repository" message with directional arrows showing both paths:

  Cherry-picking abc123 from repository (C:\r\runtime) -> VMR (C:\r\dotnet)
  

• Error handling: Wrap FindGitRoot calls in try/catch with actionable messages instead of letting exceptions bubble up cryptically.

Original prompt

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This section details on the original issue you should resolve

<issue_title>darc vmr cherry-pick does not work when not run from VMR, even with --vmr</issue_title>
<issue_description>When attempting to cherry-pick from a directory to the VMR, it is not obvious that the command must be run from the VMR directory. Failure to do so will yield a somewhat cryptic error like:

fail: Cherry-pick operation failed: Could not find file 'C:\r\runtime\src\source-mappings.json'.
dbug: System.IO.FileNotFoundException: Could not find file 'C:\r\runtime\src\source-mappings.json'.

This doesn't say what it actually means: That what it thinks it's cherry-picking to is not a VMR. The error messages in this case should be very clear, and include hints. the tool has detected that the cherry-picking operation is "starting from the VMR". It should be able to tell the user that the target (current directory is not a VMR). Better yet, it would be good for the command to say something like:

info: Cherry-picking <commit> from repository (<source>) -> VMR (<expected vmr dir>)
info: Detected source mapping name: runtime
fail: Cherry-pick operation failed: Could not find file 'C:\r\runtime\src\source-mappings.json'. Are you sure that <expected vmr dir> is the VMR? Operation should be run from VMR dir or with --vmr

Furthermore, passing --vmr does not work. For example:

e C:\r\runtime --commit 19e7456131cad3ad17cd16871f928d06e7e1b391
info: Cherry-pick operation starting from repository
info: Detected mapping name: runtime
fail: Cherry-pick operation failed: Could not find file 'C:\r\runtime\src\source-mappings.json'.
dbug: System.IO.FileNotFoundException: Could not find file 'C:\r\runtime\src\source-mappings.json'.
      File name: 'C:\r\runtime\src\source-mappings.json'
         at Microsoft.Win32.SafeHandles.SafeFileHandle.CreateFile(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options)
         at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
         at System.IO.Strategies.OSFileStreamStrategy..ctor(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
         at System.IO.Strategies.FileStreamHelpers.ChooseStrategyCore(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
         at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options, Int64 preallocationSize)
         at System.IO.File.AsyncStreamReader(String path, Encoding encoding)
         at System.IO.File.InternalReadAllTextAsync(String path, Encoding encoding, CancellationToken cancellationToken)
         at Microsoft.DotNet.DarcLib.VirtualMonoRepo.SourceMappingParser.ParseMappings(String mappingFilePath) in /_/src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/SourceMappingParser.cs:line 40
         at Microsoft.DotNet.Darc.Operations.VirtualMonoRepo.CherryPickOperation.ExecuteInternalAsync() in /_/src/Microsoft.DotNet.Darc/Darc/Operations/VirtualMonoRepo/CherryPickOperation.cs:line 84
         at Microsoft.DotNet.Darc.Operations.VirtualMonoRepo.CherryPickOperation.ExecuteAsync() in /_/src/Microsoft.DotNet.Darc/Darc/Operations/VirtualMonoRepo/CherryPickOperation.cs:line 49

This needs to be fixed.</issue_description>

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

• Fixes darc vmr cherry-pick does not work when not run from VMR, even with --vmr #6017

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[premun]

@mmitche what state is this in? Did you test it locally? Can we merge?