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/util/gemini_thinking.go

@@ -136,6 +136,12 @@ func ApplyGeminiThinkingLevel(body []byte, level string, includeThoughts *bool)
 			updated = rewritten
 		}
 	}
+	if it := gjson.GetBytes(body, "generationConfig.thinkingConfig.include_thoughts"); it.Exists() {
+		updated, _ = sjson.DeleteBytes(updated, "generationConfig.thinkingConfig.include_thoughts")
+	}
+	if tb := gjson.GetBytes(body, "generationConfig.thinkingConfig.thinkingBudget"); tb.Exists() {
+		updated, _ = sjson.DeleteBytes(updated, "generationConfig.thinkingConfig.thinkingBudget")
+	}
 	return updated
 }
 
@@ -167,6 +173,12 @@ func ApplyGeminiCLIThinkingLevel(body []byte, level string, includeThoughts *boo
 			updated = rewritten
 		}
 	}
+	if it := gjson.GetBytes(body, "request.generationConfig.thinkingConfig.include_thoughts"); it.Exists() {
+		updated, _ = sjson.DeleteBytes(updated, "request.generationConfig.thinkingConfig.include_thoughts")
+	}
+	if tb := gjson.GetBytes(body, "request.generationConfig.thinkingConfig.thinkingBudget"); tb.Exists() {
+		updated, _ = sjson.DeleteBytes(updated, "request.generationConfig.thinkingConfig.thinkingBudget")
+	}
 	return updated
 }