feat: align image generation processor with embeddings model handling patterns

- Update modelNameOverride field type from string to internalapi.ModelNameOverride for type consistency
- Implement proper original model tracking by calling SetOriginalModel with request body model before overrides

Signed-off-by: Hrushikesh Patil <[email protected]>

Author: nutanix-Hrushikesh

internal/extproc/mocks_test.go

@@ -525,7 +525,6 @@ func (m *mockImageGenerationMetrics) StartRequest(map[string]string) {}
 func (m *mockImageGenerationMetrics) SetOriginalModel(originalModel string) {
 	m.model = originalModel
 }
-
 func (m *mockImageGenerationMetrics) SetRequestModel(requestModel string) {
 	m.model = requestModel
 }