Refactor &! operator: use GetScriptBlock() directly and replace Regex with static HashSet

[Copilot]

Addresses four code review items on InvokePipelineInBackground in MiscOps.cs.

Changes

• Scriptblock branch rewrite (r2917467669 + r2917738697/729): Replaced the text-manipulation approach (brace-stripping, offset arithmetic, variable injection loop, ScriptBlock.Create(...)) with a single scriptBlockExpr.ScriptBlock.GetScriptBlock() call. The old approach risked returning a ScriptBlock object rather than executing the body. Users of { ... } &! should use explicit $using: scoping as with standard PowerShell job semantics. This also eliminates the duplicated variable-rewrite logic that existed in both branches.

• Magic variable detection (r2917738673): Replaced per-call Regex.Match(...) with a private static readonly HashSet<string> s_backgroundJobMagicVariables and a StartsWith("global:", OrdinalIgnoreCase) prefix strip. Reduces per-invocation allocations.

// Before: allocates a Regex on every variable in every backgrounded pipeline
if (!Regex.Match(variableName, "^(global:){0,1}(PID|PSVersionTable|...)$", RegexOptions.IgnoreCase).Success)

// After: O(1) lookup against a pre-built, case-insensitive HashSet
var cleanVariableName = variableName.StartsWith("global:", StringComparison.OrdinalIgnoreCase)
    ? variableName.Substring(7) : variableName;
if (!s_backgroundJobMagicVariables.Contains(cleanVariableName))

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.