Preserve Size set by type handlers for string parameters

[FirmaSpring]

Problem

When a TypeHandler is registered for string and a query is executed with an anonymous object parameter (the IL generator path in CreateParamInfoGenerator), the generated code invokes the handler's SetValue and then unconditionally sets the parameter's Size based on the string length heuristics (DbString.DefaultLength / -1), overwriting whatever Size the handler configured.

Reported in #2141. The DynamicParameters path already leaves the Size to the handler; only the IL path diverges.

Reproduction

SqlMapper.RemoveTypeMap(typeof(string));
SqlMapper.AddTypeHandler(new StringTypeHandler()); // sets parameter.Size = 12345

connection.Execute("select @p", new { p = "hello" });
// parameter.Size is 4000 on main, not 12345

Fix

Move the string Size emission into the handler is null branch, so when a type handler is registered it fully owns parameter configuration, consistent with the DynamicParameters behavior.

Testing

Added TypeHandlerSetValue_StringParameterSizeIsNotOverwritten_Issue2141 to SqliteTypeHandlerTests (in-memory SQLite, no external database required). Verified it fails on current main and passes with this change; the existing SqliteTypeHandlerTests suite passes.

Fixes #2141