Warn if Windows-style path separators (\) are used in script references

[jasonhorner]

Is your feature request related to a problem? Please describe.
I'm often frustrated when SQL project builds work on Windows but fail on Linux-based agents (e.g., ubuntu-latest in Azure DevOps) because SQLCMD-style script references use Windows path separators (e.g., :r .\Pre-Deployment\Init.sql). This causes cross-platform builds to break without a clear error message pointing to the source of the problem.

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Describe the solution you'd like
I'd like DacFx to emit a warning (e.g., SQLCMD001) during build if a script reference includes Windows-style backslashes (\) in its path. This would help developers proactively identify cross-platform compatibility issues before they hit CI/CD.

Example warning:

Warning SQLCMD001: Script reference uses Windows-style path separator: :r .\Pre-Deployment\Init.sql. Consider using forward slashes (/) for cross-platform compatibility.

The warning could be:

• Non-breaking (does not fail the build)
• Suppressible

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Describe alternatives you've considered

• Writing pre-build PowerShell or Bash scripts to scan for :r .\ and manually fail the build
• Using linting tools (but none currently cover SQLCMD path conventions well)

None of these provide the integrated, first-class experience that a native DacFx warning would.

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Additional context
This issue becomes especially painful when moving SQL build pipelines to Linux agents or Docker-based environments where path compatibility is stricter. Emitting a simple warning could save hours of troubleshooting in larger teams.

[ErikEJ]

I will add this proposal to our MsBuild.Sdk:SqlProj project!