Reduce allocations in LicenseExpressionTokenizer.HasValidCharacters by caching Regex instance

[nareshjo]

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• Issue: LicenseExpressionTokenizer.HasValidCharacters() constructs a new Regex(...) on every invocation using a hardcoded constant pattern string. Each Regex constructor call triggers internal parsing that allocates RegexParser, RegexTree, RegexNode, RegexCharClass, RegexCode, StringBuilder, and other objects (~12–15 short-lived objects per call).

  This method is called once per license expression parse via the call chain: SearchObject.ProcessSearchResultsAsync → PackageSearchMetadataCacheItem → PackageSearchMetadataContextInfo.Create → PackageSearchMetadata.get_LicenseMetadata → NuGetLicenseExpressionParser.Parse → GetTokens → LicenseExpressionTokenizer.HasValidCharacters.
  For a typical NuGet package search, this produces hundreds of Regex constructions — each allocating ~12–14 short-lived internal objects — contributing to measurable GC pressure on a path that the allocation telemetry specifically flagged.

  Allocation site	Original (per search)	After fix (per search)
  new Regex(...) internal objects	~12–15 × N (N = license parses)	1 (static, once per process)

  This matches the allocation stack trace showing StringBuilder allocated inside Regex..ctor called by HasValidCharacters during NuGet package search:

nuget.packagemanagement.visualstudio.dll!SearchObject+<ProcessSearchResultsAsync>d__.MoveNext
nuget.packagemanagement.visualstudio.dll!SearchObject.CacheBackgroundData
nuget.packagemanagement.visualstudio.dll!PackageSearchMetadataCacheItem..ctor
nuget.packagemanagement.visualstudio.dll!PackageSearchMetadataCacheItem.GetVersionInfoContextInfoAsync
nuget.visualstudio.internal.contracts.dll!VersionInfoContextInfo+<CreateAsync>d__.MoveNext
nuget.visualstudio.internal.contracts.dll!PackageSearchMetadataContextInfo.Create
nuget.protocol.dll!PackageSearchMetadata.get_LicenseMetadata
nuget.packaging.dll!NuGetLicenseExpressionParser.Parse
nuget.packaging.dll!NuGetLicenseExpressionParser.GetTokens
nuget.packaging.dll!LicenseExpressionTokenizer.HasValidCharacters
└── system.dll!Regex..ctor
    └── system.dll!RegexParser.Parse → RegexParser.ScanCharClass
        └── RegexCharClass..ctor → TypeAllocated!System.Text.StringBuilder

• Issue type: Reduce repeated identical allocations from per-call Regex construction on a hot path

• Proposed fix: Promote the per-call Regex local variable to a private static readonly Regex field with RegexOptions.CultureInvariant. The pattern and matching behavior are identical; Regex.IsMatch is documented thread-safe on constructed instances. RegexOptions.Compiled is intentionally omitted — the pattern is a trivial single character-class match where interpreted mode is sub-microsecond, and Compiled would add a non-collectible DynamicMethod on .NET Framework 4.7.2 with no measurable benefit for this pattern complexity and call frequency.

  This follows the existing convention in the codebase: PackageIdValidator.IdRegex uses the identical private static readonly Regex pattern for package ID validation.

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