feat(amp): enhance model mapping and Gemini thinking configuration

This commit introduces several improvements to the AMP (Advanced Model Proxy) module:

- **Model Mapping Logic:** The `FallbackHandler` now uses a more robust approach for model mapping. It includes the extraction and preservation of dynamic "thinking suffixes" (e.g., `(xhigh)`) during mapping, ensuring that these configurations are correctly applied to the mapped model. A new `resolveMappedModel` function centralizes this logic for cleaner code.
- **ModelMapper Verification:** The `ModelMapper` in `model_mapping.go` now verifies that the target model of a mapping has available providers *after* normalizing it. This prevents mappings to non-existent or unresolvable models.
- **Gemini Thinking Configuration Cleanup:** In `gemini_thinking.go`, unnecessary `generationConfig.thinkingConfig.include_thoughts` and `generationConfig.thinkingConfig.thinkingBudget` fields are now deleted from the request body when applying Gemini thinking levels. This prevents potential conflicts or redundant configurations.
- **Testing:** A new test case `TestModelMapper_MapModel_TargetWithThinkingSuffix` has been added to `model_mapping_test.go` to specifically cover the preservation of thinking suffixes during model mapping.

internal/api/modules/amp/model_mapping_test.go

@@ -71,6 +71,25 @@ func TestModelMapper_MapModel_WithProvider(t *testing.T) {
 	}
 }
 
+func TestModelMapper_MapModel_TargetWithThinkingSuffix(t *testing.T) {
+	reg := registry.GetGlobalRegistry()
+	reg.RegisterClient("test-client-thinking", "codex", []*registry.ModelInfo{
+		{ID: "gpt-5.2", OwnedBy: "openai", Type: "codex"},
+	})
+	defer reg.UnregisterClient("test-client-thinking")
+
+	mappings := []config.AmpModelMapping{
+		{From: "gpt-5.2-alias", To: "gpt-5.2(xhigh)"},
+	}
+
+	mapper := NewModelMapper(mappings)
+
+	result := mapper.MapModel("gpt-5.2-alias")
+	if result != "gpt-5.2(xhigh)" {
+		t.Errorf("Expected gpt-5.2(xhigh), got %s", result)
+	}
+}
+
 func TestModelMapper_MapModel_CaseInsensitive(t *testing.T) {
 	reg := registry.GetGlobalRegistry()
 	reg.RegisterClient("test-client2", "claude", []*registry.ModelInfo{