chore(sonar)!: mark Markdown generator classes as 'public static class' (S1118)

After the CA1822 cascade in #15 left every method on the eight
Markdown generator classes static, the classes themselves were
instantiable shells that consumers couldn't usefully `new`. This
commit flips the `class` modifier to `static class` on all eight:

- `Markdown.SqlServer.QueryBreakdownGenerator`
- `Markdown.SqlServer.SqlStatementGenerator`
- `Markdown.LinqToSql.QueryBreakdownGenerator`
- `Markdown.LinqToSql.SqlStatementGenerator`
- `Markdown.PostgreSql.QueryBreakdownGenerator`
- `Markdown.PostgreSql.SqlStatementGenerator`
- `Markdown.Snowflake.QueryBreakdownGenerator`
- `Markdown.Snowflake.SqlStatementGenerator`

Test fixtures drop the now-meaningless `_generator = new …()` field
and `[SetUp]` (kept the existing Setup body where unrelated state was
also initialized — `QueryMarkdownGenerationTests` and
`LinqToSql.QueryBreakdownGeneratorTests`).

BREAKING CHANGE: External NuGet consumers can no longer write
`new Markdown.Snowflake.SqlStatementGenerator()` (or any of the other
seven classes above) — the type is now a static container and may only
be referenced by name, e.g.
`Markdown.Snowflake.SqlStatementGenerator.GenerateSequenceDiagram(…)`.
The call syntax for the static methods is unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/Strata.SqlTools.Markdown.Tests/Snowflake/QueryBreakdownGeneratorTests.cs

@@ -6,14 +6,6 @@ namespace Strata.SqlTools.Markdown.Tests.Snowflake;
 [TestFixture]
 public class QueryBreakdownGeneratorTests
 {
-    private QueryBreakdownGenerator _generator = null!;
-
-    [SetUp]
-    public void Setup()
-    {
-        _generator = new QueryBreakdownGenerator();
-    }
-
     [Test]
     public void GenerateMermaidDiagram_SimpleSnowflakeQuery_GeneratesValidMermaid()
     {

tests/Strata.SqlTools.Markdown.Tests/Snowflake/SqlStatementGeneratorTests.cs

@@ -6,14 +6,6 @@ namespace Strata.SqlTools.Markdown.Tests.Snowflake;
 [TestFixture]
 public class SqlStatementGeneratorTests
 {
-    private SqlStatementGenerator _generator = null!;
-
-    [SetUp]
-    public void Setup()
-    {
-        _generator = new SqlStatementGenerator();
-    }
-
     [Test]
     public void GenerateSequenceDiagram_SnowflakeQuery_GeneratesValidDiagram()
     {