[WIP] Fix copilot setup steps for .NET SDK installation

[Copilot]

• Analyzed the repository structure and identified the issue
• Found that copilot-setup-steps.yml uses incorrect parameter (global-json-file instead of dotnet-version)
• Verified that other workflow files (ci-build-test.yml, release.yml) correctly use dotnet-version parameter
• Update copilot-setup-steps.yml to use dotnet-version: 10.0.100-rc.1.25451.107
• Test the changes by verifying the workflow file syntax
• Document the fix

Original prompt

Please fix the copilot setup steps file. I need it to install the right .NET SDK (currently .NET 10 RC1), but it's not doing so.

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Copilot]

@stephentoub Unfortunately, I hit an error while trying to use the custom Copilot setup steps configured for this repository and had to close this PR. The error I am seeing is:

no valid jobs found in copilot-setup-steps.yaml

Once you or someone with the necessary access fixes the problem, please unassign and then reassign issue #0 to me and I'll retry. Thanks!