Improve version maintenance agentic workflow

[ViktorHofer]

Fixes issues from the first run of the version maintenance workflow (https://github.com/dotnet/maintenance-packages/actions/runs/22864461699):

Changes

• Fix NuGet API calls - Use curl instead of Python's urllib.request for proxy/SSL compatibility in the sandboxed AWF container. The previous run failed to update PackageValidationBaselineVersion because urllib.request couldn't complete SSL handshakes through the Squid CONNECT-tunnel proxy.
• Add runtimes - Declare Python 3.x and .NET 8.0 in the workflow frontmatter (replaces copilot-setup-steps.yml).
• Fix VersionPrefix logic - Unconditional VersionPrefix is now set to the latest NuGet version (same as PackageValidationBaselineVersion); conditional VersionPrefix with Condition is patch-incremented as before.
• Conditional AssemblyVersion bumps - Only increment AssemblyVersion when a VersionPrefix in the same file was actually changed.
• Schedule - Changed from daily to weekly.
• PR body formatting - Use package names instead of full file paths in the summary table.

[ericstj]

This is a whole lot more text for the simple rule of "If IsPackable is true, and the Packable version exists on nuget, set IsPackable to false and increment all versions within the servicing property group in the same file". Can we try leaving it minimal and just referencing a "good" example commit?

[ViktorHofer]

Feel free to push into this PR with a simplified version. I didn't write this myself, this is the output of an LLM prompt. Most other agentic workflows that we are building in other repos also tend to be more verbose.

[ericstj]

It's OK if it works, just more verbose than I'd prefer. I remember when you pushed for this versioning scheme when creating maintenance-packages you had in mind a very simple set of rules for incrementing.

[ViktorHofer]

These are instructions for an LLM running as part of gh agentic workflows under a restrictive environment - not for a human. Most of the prompt is handling these special requirements, i.e. providing a script and explaining how to handle the firewall section. The actual rules are roughly 10 sentences.