fix(ci): use backslash paths for msiexec in WinFsp install

[FSM1]

Summary

• The WinFsp MSI install step was failing on Windows CI because msiexec.exe treats / as a flag prefix — the forward slashes from github.workspace string concatenation caused "This installation package could not be opened"
• Use Join-Path for proper Windows backslash paths, quote the path in msiexec arguments, and check the exit code with -PassThru

Test plan

• Merge and push a staging tag to trigger the deploy-staging workflow
• Verify the Windows desktop build job passes (WinFsp installs successfully)

🤖 Generated with Claude Code

Summary by CodeRabbit

• Chores
  • Enhanced deployment process stability with improved Windows file path handling and more robust MSI installation error detection.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 7bbd7bc and c46e07b.

📒 Files selected for processing (1)

• .github/workflows/deploy-staging.yml

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Walkthrough

This pull request modifies the Windows MSI installation workflow step in the deployment script. It replaces hard-coded path strings with PowerShell's Join-Path cmdlet for proper Windows path construction and enhances MSI installation robustness by capturing process handles with PassThru, properly quoting the MSI path, and validating exit codes.

Changes

Cohort / File(s)	Summary
GitHub Workflow MSI Installation .github/workflows/deploy-staging.yml	Replaces hard-coded Windows paths with Join-Path for correct path construction, adds quoted MSI path to msiexec invocation with PassThru process capture, and implements exit code validation for failure detection. Preserves existing hash verification and directory creation logic.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly related PRs

• fix(ci): write WinFsp registry key for winfsp-sys build script #234: Also modifies Windows MSI install steps in the same workflow file, adding WinFsp registry writes and verification logic following MSI installation.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately and concisely describes the main change: fixing CI by using backslash paths for msiexec in WinFsp installation, which is the primary issue addressed in the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/winfsp-msi-path

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