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.go

@@ -59,7 +59,8 @@ func (m *DefaultModelMapper) MapModel(requestedModel string) string {
 	}
 
 	// Verify target model has available providers
-	providers := util.GetProviderName(targetModel)
+	normalizedTarget, _ := util.NormalizeThinkingModel(targetModel)
+	providers := util.GetProviderName(normalizedTarget)
 	if len(providers) == 0 {
 		log.Debugf("amp model mapping: target model %s has no available providers, skipping mapping", targetModel)
 		return ""