Address review feedback: fix stderr handling, validation, and editorconfig

[Copilot]

Description

Addresses unresolved review comments from #14400:

• .editorconfig: Removed root = true to inherit repo-level formatting rules (whitespace, charset). Follows pattern in tools/scripts/.editorconfig, playground/.editorconfig, src/Vendoring/.editorconfig.

• Directory.Packages.props: Added comment explaining Hex1b.Tool is tracked for central version management despite only being referenced in .config/dotnet-tools.json.

• CLI argument validation: Added bounds checks, replaced int.Parse with int.TryParse, validated positive values, surfaced friendly errors:

  $ dotnet MigratePackage.cs pkg 1.0.0 --poll-interval
  [ERROR] Missing value for --poll-interval. Expected a positive integer (seconds).
  
  $ dotnet MigratePackage.cs pkg 1.0.0 --poll-interval -5
  [ERROR] Invalid value for --poll-interval: '-5'. Expected a positive integer (seconds).

• RunProcessAsync stderr handling: Read stdout/stderr concurrently to prevent deadlocks when child processes write large stderr output. Return stderr content on failure for diagnostics. Await tasks directly instead of Task.WhenAll + .Result.

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
  • No
• Does the change require an update in our Aspire docs?
  • Yes
  • No

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