refactor(core): change GetGeneratedNamespace to NoInlining for error paths

**Issue:**
GetGeneratedNamespace() was marked with AggressiveInlining but is only
called in exception/error paths (when serializers are not registered).

**Problem:**
- Method allocates a StringBuilder and performs string manipulation
- Inlining this into every error-throwing call site bloats code size
- Error paths should be optimized for smaller code, not speed
- Misleading to mark error-path helpers as AggressiveInlining

**Solution:**
Changed `MethodImpl(MethodImplOptions.AggressiveInlining)` to
`MethodImpl(MethodImplOptions.NoInlining)` with clarifying comment.

**Benefits:**
- Smaller code size at call sites (10 call sites across NinoSerializer/NinoDeserializer)
- Clearer intent - this is an error path helper
- Consistent with other error path methods (e.g., ThrowInvalidCast in NinoDeserializer.cs:302)
- No performance impact (only called when exceptions are thrown)

**Locations using this method (all error paths):**
- NinoSerializer.cs: 154, 234, 268, 298
- NinoDeserializer.cs: 102, 148, 361, 384, 432, 455

This is a code quality improvement with no runtime performance impact
in the success case.

Author: claude

src/Nino.Core/NinoHelper.cs

@@ -12,8 +12,9 @@ internal static class NinoHelper
         /// <summary>
         /// Gets the generated namespace for a type's assembly.
         /// This must match the logic in NinoTypeHelper.GetNamespace()
+        /// Called only in error paths - intentionally not inlined to keep call sites small.
         /// </summary>
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        [MethodImpl(MethodImplOptions.NoInlining)]
         internal static string GetGeneratedNamespace(Type type)
         {
             var assemblyName = type.Assembly.GetName().Name ?? string.Empty;