Fix warmup regression: avoid per-expression AppDomain assembly scan (fixes #739)

[benluersen]

Problem

Since 71d59dc (#675), CustomTypeProvider.GetCustomTypes() includes base.GetCustomTypes(), which scans every AppDomain assembly for [DynamicLinqType] types. System.Linq.Dynamic.Core caches that scan only per provider instance, and RuleExpressionParser.Parse builds a new ParsingConfig + CustomTypeProvider for every expression parsed (Dynamic LINQ's KeywordsHelper enumerates the full type set on each ExpressionParser construction). The result is one full assembly scan per expression, per rule.

For large workflows this regressed warmup ~6.6x vs 5.0.3 (113.8 s vs 17.3 s for 20,000 rules with local params and 174 loaded assemblies). The compiled-delegate cache from #727 does not help because each rule expression is unique. This is the remaining root cause behind #707.

Change

• RuleExpressionParser: reuse one ParsingConfig/CustomTypeProvider across parses; rebuild only when ReSettings.CustomTypes is swapped (AutoRegisterInputType replaces the array on workflow registration), preserving the behavior introduced by [Bug Fix] Handling of Automatic Type Registration #675.
• CustomTypeProvider: memoize the merged custom-type set; it is fixed after construction but was rebuilt — including the assembly scan — on every GetCustomTypes() call.

Results

Same 20,000-rule benchmark: 113.8 s → 16.3 s, matching 5.0.3, with identical rule results.

All 170 existing unit tests pass. No public API change.

[benluersen]

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