feat(thinking): add xAI provider support with reasoning.effort implementation

- Implemented `xAI` provider for thinking configurations with support for reasoning.effort levels.
- Registered `xAI` in available providers and updated relevant APIs for compatibility.
- Added unit tests for `xAI` provider functionality, including fallback logic for unsupported levels.
- Integrated `xAI` with executor handling and ensured conformance with OpenAI-compatible standards.

internal/thinking/strip.go

@@ -42,7 +42,7 @@ func StripThinkingConfig(body []byte, provider string) []byte {
 			"reasoning_effort",
 			"thinking",
 		}
-	case "codex":
+	case "codex", "xai":
 		paths = []string{"reasoning.effort"}
 	default:
 		return body