Resolves SonarQube S2696 in CommandVisitor.cs.
The static _parameterIndex field was mutated from an instance method,
causing every new CommandVisitor to inherit the previous instance's
counter and never reset. Parameter indices now restart at $1 per
visitor, which is the intended PostgreSQL behavior.
Adds CommandVisitorTests.TwoVisitors_HaveIndependentParameterIndices to
lock in the contract via reflection (FormatParameterName is protected).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
