Fix thread-safety bug in QueryGenerator caused by static mutable field

[penspanic]

Problem

QueryGenerator uses a static Dictionary<ISymbol, List<IMethodSymbol>> _classToMethods field that is shared across all concurrent generator invocations. During parallel builds (e.g. multiple projects referencing the generator, or multi-targeting), this causes intermittent:

Generator 'QueryGenerator' failed to generate source. It will not contribute to the output
and compilation errors may occur as a result. Exception was of type 'InvalidOperationException'
with message 'Collection was modified; enumeration operation may not execute.'

The error occurs because one thread enumerates _classToMethods in the foreach loop while another thread reassigns or mutates it in a concurrent Generate() call.

Fix

Remove the static field and replace it with a local variable inside Generate(), passed as a parameter to AddMethodToClass(). Since the dictionary is already re-created at the start of every Generate() call, there is no need for it to persist across calls.

Test

Added ConcurrencyTest that runs 8 parallel generator invocations with [Repeat(10)]. Before the fix, this fails 100% of the time. After the fix, it passes 100%.