Fix the regression in tab completing positional parameters - #27857

PR Summary

The PR #17796 and #18755 introduced a regression in tab completion -- dir | % <tab> completes with member names of FileInfo object before that change (v7.2 v7.3 behavior), but after that PR, it completes with file names within the current directory (v7.4 + behavior).

This is because the original code that handled positional parameter completion was able to preserve an alternative non-default-set positional parameter, which it would fall back when the default-set positional parameter had no completion result. For ForEach <tab>, the default-set positional parameter applicable for the position is -Process [ScriptBlock], which has no completion result, so it falls back to the non-default-set parameter -MemberName [string].

However, the PR #17796 + #18755 removed the alternative positional parameter preservation, so for ForEach <tab>, it will complete for -Process [ScriptBlock] only. That caused the regression.

Improvements to positional parameter completion

The implementation before #17796 + #18755 is also inconsistent:

1. Only 1 non-default-set parameter is preserved. That means if there are more than 1 non-default parameter sets and all contain positional parameter with the same position, only one parameter of them will be considered in completion, depending on the discovery order, and the rest will be ignored.
2. If we only find a non-default set parameter, then we will not fall back to the remaining-argument parameters no matter that parameter has completion results or not; but if we only find a default-set parameter, we will fall back to the remaining-argument parameters if there is no completion result.

This pull request improves the logic for positional parameter completion in PowerShell. It fixed the regression and those inconsistencies. It keeps the improvement by #17796 + #18755 -- find the positional parameter with the closest position instead of the exact position. In addition, it prioritizes the default-set positional parameter and preserve non-default-set parameters with the same best position.

It refactored the logic in CompletePositionalArgument (in CompletionCompleters.cs) to:

• Find all parameters at the closest position across valid parameter sets, prioritizing the default set when available, but falling back to alternatives if needed.
• Continue through alternative parameters for completions if the default-set parameter yields no results. Process all candidates at the best position until a completion is found.
• Only fall back to remaining-argument parameters when no applicable positional parameter is found.

Enhanced test coverage

Added new tests in TabCompletion.Tests.ps1 to verify:

• Fallback to positional parameters from alternative parameter sets when the default set has no completions.
• Iterative attempts across parameter sets until completions are found.
• Correct completion behavior for parameters declared at different positions in different parameter sets, with and without a default parameter set.

PR Checklist

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• Breaking changes
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• Testing - New and feature
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  • Make sure you've added a new test if existing tests do not effectively test the code changed