fix(pgsql): make CommandVisitor parameter index instance-scoped

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>

src/Strata.SqlTools.PostgreSql/Visitors/CommandVisitor.cs

@@ -9,7 +9,7 @@ namespace Strata.SqlTools.Visitors.PostgreSql;
 /// </summary>
 public class CommandVisitor : SqlServerCommandVisitor
 {
-    private static int _parameterIndex = 1;
+    private int _parameterIndex = 1;
 
     /// <summary>
     /// Formats an identifier for PostgreSQL using double-quote quoting.

tests/Strata.SqlTools.PostgreSql.Tests/PostgreSql/CommandVisitorTests.cs

@@ -0,0 +1,34 @@
+using Strata.SqlTools.Visitors.PostgreSql;
+
+namespace Strata.SqlTools.PostgreSql.Tests.PostgreSql;
+
+[TestFixture]
+public class CommandVisitorTests
+{
+    [Test]
+    public void TwoVisitors_HaveIndependentParameterIndices()
+    {
+        var visitor1 = new CommandVisitor();
+        var visitor2 = new CommandVisitor();
+
+        var param1a = InvokeFormatParameterName(visitor1, "p");
+        var param1b = InvokeFormatParameterName(visitor1, "p");
+        var param2a = InvokeFormatParameterName(visitor2, "p");
+
+        Assert.That(param1a, Is.EqualTo("$1"), "first visitor's first parameter should be $1");
+        Assert.That(param1b, Is.EqualTo("$2"), "first visitor's second parameter should be $2");
+        Assert.That(param2a, Is.EqualTo("$1"), "second visitor must start at $1, not inherit visitor1's counter");
+    }
+
+    private static string InvokeFormatParameterName(CommandVisitor visitor, string name)
+    {
+        var method = typeof(CommandVisitor).GetMethod(
+            "FormatParameterName",
+            System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Public)
+            ?? typeof(Strata.SqlTools.Visitors.SqlServer.CommandVisitor).GetMethod(
+                "FormatParameterName",
+                System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
+        Assert.That(method, Is.Not.Null, "FormatParameterName must exist as a protected method on the visitor hierarchy");
+        return (string)method!.Invoke(visitor, new object[] { name })!;
+    }
+}